LIBRARY  OF 

.  B.  GRISWOLD. 


^~ 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


GIFT  OF 

Mrs.  Franziska  Roos 


A  POPULAR 


TREATISE   ON  COMETS, 


BY 

JAMES    C.  WATSON,  A.M., 

FORMERLY   PROFESSOR  OF  ASTRONOMY,  AND  NOW  PROFESSOR  OF  PHYSICS,  UT 
TEX  UNIVERSITY  OF  MICHIGAN. 


ER1T  QUI  DEMONSTRET  ALIQUAXDO,  IN  QUIBU8  COOTS  PARTIBUS  CURRANT, 
CCM  TAX   SEDUCTI  A  CETERIS   ERRENT,  ET  QCANTI  QUALESQUE  SUIT. 

Seneca,  Quasi.  Nat^  lib.  rii.,  c.  27. 


PHILADELPHIA: 
JAMES     CHALLEN    &    SON. 

DETROIT :    RAYMOND    &    LAPHAM.      CHICAGO :    S.  C.  GRIGGS    &   CO. 

ANN  ARBOR:  SCHOFF  t  MILLER. 
1861. 


Entered,  according  to  Act  of  Congress,  in  the  year  1861,  by 

JAMES  C.  WATSON, 

in  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the  Eastern 
District  of  Pennsylvania. 

STEREOTYPED  BT  J.  FAGAN FEINTED  BY   I.  ASUMEAD. 


Astronomy 
GIFT 


NN33 


TO 

E.   B.  WARD,  ESQ., 

OF    DETROIT, 

AS   A   TESTIMONIAL  OF   THE   LIVELY   INTEREST   WHICH   HE   ! 
TAKEN    IN    THE     PROMOTION    OF    THE    STUDY  OF    THE 
PHYSICAL     SCIENCES,    THIS     WORK     IS     MOST 
RESPECTFULLY   INSCRIBED,    BY   THE 

AUTHOR. 


2C6 


PREFACE. 


THE  announcement  of  the  discovery  of  a  comet  has 
always  been  received  with  great  interest  by  both  the 
learned  and  unlearned.  By  the  former,  it  is  hailed 
with  delight,  as  affording  new  proofs  of  the  perfection 
of  the  laws  which  regulate  the  motions  of  the  celestial 
bodies ;  while  to  the  latter,  the  appearance  of  a  comet 
of  any  considerable  magnitude  furnishes,  too  often,  the 
basis  of  the  most  foolish  and  superstitious  fears,  being 
regarded  as  the  forerunner  of  pestilence,  famine,  or 
some  other  dreadful  calamity. 

The  time,  however,  has  now  come  when  all  such 
fears  should  be  at  an  end,  and  it  has  been  for  the  pur- 
pose of  disseminating  correct  notions  in  regard  to  the 
nature  and  physical  constitution  of  these  wonderful 
bodies,  among  those  not  versed  in  astronomical  science, 
that  this  volume  has  been  prepared.  The  subject  is 
here  treated  in  the  most  complete  and  candid  manner, 
and  all  technicalities  are  studiously  avoided.  In  a 
word,  the  object  has  been  to  convey  to  every  intelligent 
reader  all  the  knowledge  in  regard  to  the  motions  and 
character  of  these  erratic  bodies  possessed  by  the  pro- 
fessional astronomer  of  the  present  day,  without  making 
it  necessary  to  introduce  the  symbolic  operations  of 
mathematical  analysis. 

(vii) 


viii  PREFACE. 

To  accomplish  this,  it  has  been  necessary  in  some 
instances  to  state  facts  without  giving  the  processes 
which  astronomers  have  employed  in  their  determina- 
tion ;  but  the  statements  which  have  been  made  may 
be  relied  on  as  being  accurate.  Whenever  the  strictly 
popular  character  of  the  work  has  permitted  a  satis- 
factory explanation  of  the  practical,  as  well  as  the 
theoretical,  details  of  the  calculations,  it  has  been 
attempted. 

Although  it  may  be  said  that  the  age  in  which  we 
live  is  indeed  distinguished  for  the  clearest  and  most 
enlarged  views  of  social  and  political  science,  yet  it 
is  not  less  marked  by  the  disposition,  so  unequivocally 
and  universally  manifested,  to  reject  the  inordinate 
estimate  heretofore  placed  upon  merely  ornamental 
literature;  and  while  it  does  not  refuse  their  jusi  rank 
and  influence  to  such  studies,  it  admits  of  that  high 
consideration  to  which  they  are  entitled,  the  sciences 
which  explain  the  beautiful  phenomena  of  the  physical 
world.  It  is  therefore  encouraging  to  know  that  the 
demand  for  some  portion  of  scientific  knowledge,  and 
the  desire  to  be  informed  of  what  is  passing  in  that 
universe  of  which  our  earth  is  so  minute  and  appa- 
rently insignificant  a  member,  no  longer  confined  within 
the  sacred  precincts  of  astronomical  observatories  and 
academies  of  science,  has  spread  throughout  the  whole 
extent  of  civilized  society.  To  supply  wants  thus 
created,  so  far  as  they  relate  to  those  beautiful  and 
mysterious  worlds  known  as  comets,  this  book  is  now 
offered  with  the  hope  that  it  may  merit  the  approbation 
of  an  enlightened  public. 

J.  C.  W. 
ANN  ARBOR,  MICHIGAN,  September,  1860. 


CONTENTS. 


CHAPTER  I. 

Introductory  remarks  —  Characteristic  difference  between 
comets  and  planets  —  Superstitious  fears  excited  by  the 
appearance  of  comets  —  Opinions  held  by  the  ancients  — 
Fears  excited  in  the  middle  ages  and  at  the  present  day — 
Influence  of  comets  on  the  weather  —  The  great  dry  fogs 
of  1783  and  1831  —  Possibility  of  a  collision  between  a 
comet  and  the  earth  —  Effect  of  such  a  collision  —  True 
theory  of  cometary  influences PAGE  13 

CHAPTER   II. 

Apparent  forms  of  comets — Description  of  some  of  the  most 
remarkable  comets  which  have  appeared  during  the  period 
of  authentic  history  — The  great  comets  of  1556,  1680, 
1769,  1807,  1811,  1843,  1853,  and  1858  — Number  of 
comets 77 

CHAPTER  III. 

Orbits  of  comets  —  Computation  of  an  orbit  —  Ephemeris  of 
a  comet  —  Periodic  comets  —  Halley's  comet  —  Encke's 
comet  —  Biela's  comet — Faye's  comet  —  De  Vico's  comet 
— Brorsen's  comet  —  D' Arrest's  comet  —  Tuttle's  comet 
—  Winnecke's  comet  —  LexelFs  comet  of  1770  — Other 
periodic  comets  —  Relation  supposed  to  exist  between  the 
comets  of  short  period  and  the  asteroid  planets  —  Possi- 
bility of  a  collision  between  a  comet  and  planet — Probable 
appulse  of  two  comets 149 


x  CONTENTS. 

CHAPTER  IV. 

Physical  constitution  of  comets — Perturbations  of  the  comets 
by  the  planets  —  The  masses  of  comets  —  Effect  of  intense 
heat  and  cold  on  their  form  —  Light  of  comets — Tails  of 
comets  —  Theory  of  the  formation  of  the  tail  —  Influence 
of  the  tail  on  the  motion  of  the  nucleus  in  its  orbit  — 
Origin  of  comets  —  Formation  of  new  worlds  —  Nebular 
hypothesis 273 

CHAPTER  V. 

On  the  existence  of  a  resisting  medium  in  space  —  Character 
of  the  resisting  medium  —  Its  effect  on  the  motions  of  the 
heavenly  bodies  —  The  stability  of  the  solar  system  — 
Influence  of  the  comets  —  General  remarks 333 


A 

POPULAR   TREATISE 

ON 

COMETS. 


CHAPTER  I. 

INTRODUCTORY    REMARKS CHARACTERISTIC    DIFFERENCE    BETWEEN 

COMETS  AND  PLANETS SUPERSTITIOUS  FEARS  EXCITED  BY  THE  AP- 
PEARANCE OF  COMETS OPINIONS  HELD  BY  THE  ANCIENTS FEARS 

EXCITED  IN  THE  MIDDLE  AGES  AND  AT  THE  PRESENT  DAY  —  INFLU- 
ENCE OF  COMETS  ON  THE  WEATHER  —  THE  GREAT  DRY  FOGS  OF  1783 

AND  1831 POSSIBILITY  OF  A  COLLISION    BETWEEN  A    COMET    AND 

THE    EARTH EFFECT   OF    SUCH    A   COLLISION TRUE    THEORY   OF 

COMETARY    INFLUENCE. 

THERE  is  no  department  of  human  learning  and 
research  in  which  the  mind  has  been  exercised  with 
more  transcendant  success  than  in  the  theory  of 
astronomy.  The  development  of  the  legitimate 
results  of  the  theory  of  gravitation,  first  discovered 
by  Newton,  and  its  extension  to  the  far  distant 
regions  of  the  Universe,  has  enabled  the  astronomer 
of  the  present  day  to  comprehend,  at  one  glance,  as 
it  were,  the  past,  the  present,  and  the  future  changes 
of  entire  systems.  It  has  enabled  him  to  show  that 
'2  (13) 


14  TREATISE   ON    COMETS. 

even  in  our  own  solar  system,  with  its  planets  and 
their  satellites,  where  everything  is  in  a  state  of 
motion  apparently  the  most  complex  that  could  be 
devised,  although  the  mutual  attraction  of  the 
bodies  which  compose  it,  causes  the  orbits  alter- 
nately to  contract  and  expand,  and  their  planes  to 
rock  slowly  up  and  down,  yet,  that  stability  is  there. 
It  permits  him  also  to  declare  that  this  unseen  agent 
of  the  Supreme  Intelligence,  mysterious  in  its  nature 
as  spirit  itself,  connects  the  parts  of  the  universe  so 
intimately,  that  action  is  answered  instantaneously 
by  reaction  through  distances  which  elude  even  the 
grasp  of  the  imagination ;  and  yet  the  law  of  this 
force,  though  the  most  general  and  exalted  that  man 
has  discovered,  is  so  simple  that  the  effects  of  gravi- 
tation, however  numerous  and  complicated,  have 
been,  or  may  be,  predicted  with  unerring  certainty. 
Thus  do  we  find  that  as  far  as  telescopic  vision  can 
extend,  in  the  most  remote  realms  of  space,  sun 
revolves  around  sun,  and  system  around  system,  in 
obedience  to  the  same  power  that  causes  the  rains 
to  descend  and  the  tides  to  flow. 

If  we  go  back  to  the  earliest  ages  of  the  world, 
and  trace  forward  the  progress  of  astronomical  dis- 
covery until  the  time  of  Kewton,  we  shall  find  that, 
notwithstanding  the  perseverance  and  acuteness  of 
the  many  illustrious  astronomers  who  preceded  him, 
there  were  many  phenomena  presented  in  the  hea- 


COMETS   AND  PLANETS.  15 

vens  which,  with  their  knowledge  of  laws  and  forces, 
baffled  every  attempt  at  explanation,  but  which  now 
present  us  only  with  striking  proofs  of  the  operations 
of  the  attraction  of  gravitation.  The  planets,  which 
were  never  so  distant  as  to  escape  vision,  having 
perpetually  afforded  opportunities  for  observation, 
were  known  to  move  in  obedience  to  some  definite 
law  of  nature,  no  matter  what  that  law  might  be. 
But  there  was  another  class  of  celestial  bodies  whose 
motions  seemed  to  be  at  variance  with  any  force 
which  could  harmonize  with  the  actual  motions  of 
the  planets;  and  these,  from  the  circumstance  of 
their  having  presented,  in  most  instances,  the  phe- 
nomenon of  a  bright  head  or  nucleus,  with  a  train 
of  fainter  light  bearing  some  resemblance  to  a  tuft 
or  lock  of  hair,  were  called  comets  (from  the  Latin 
word  coma,  signifying  a  bunch  of  hair),  in  order  to 
distinguish  them  from  the  other  luminaries,  which, 
whether  near  or  remote,  apparently  fixed  or  mova- 
ble, never  exhibited  any  similar  appearance.  The 
planets,  too,  were  known  to  move  regularly,  in  their 
annual  course,  from  west  to  east,  in  paths  which 
were  nearly  circular,  and  which  were  confined  to  a 
very  narrow  belt  or  zone  of  the  heavens ;  and  con- 
sequently they  were  always  visible,  except  when  in 
the  immediate  vicinity  of  the  sun.  But  the  case 
presented  by  the  comets  was  far  different.  They 
were  seen  only  for  a  brief  period,  and  were  observed 


16  TREATISE   ON   COMETS. 

to  move  over  a  wide  extent  of  the  heavens,  and  in 
every  direction  with  reference  to  the  path  pursued 
by  the  earth.  Some  were  found  to  move  in  orbits 
whose  planes  must  be  nearly  at  right  angles  with 
the  orbits  of  the  planets,  and  others  were  found  to 
move  nearly  in  their  plane.  Some  were  seen  to 
move  from  west  to  east,  in  the  direction  of  the 
planetary  motions,  while  an  equal  number  retro- 
graded, or  from  east  to  west ;  and  from  the  appear- 
ances which  they  presented  from  day  to  day,  it  was 
evident  that  they  approached  very  near  the  sun,  and 
then  retreated  to  the  most  remote  regions  of  space. 
It  was  without  doubt  this  mystery  which  attended 
their  motions,  as  well  as  that  of  their  physical  cha- 
racter, which  has  in  all  ages  produced  that  sensation 
which  we  shall  presently  notice.  The  planets  being 
always  present,  —  before  the  invention  of  the  tele- 
scope,—  attracted  no  very  considerable  attention; 
but  the  comets,  by  their  unusual  aspect,  their  sud- 
den arrival,  and  the  prodigious  velocity  with  which 
they  winged  their  flight  through  our  system  into  the 
deep  recesses  of  the  heavens,  excited  the  most  vivid 
impressions  on  the  imagination.  "We  shall  find, 
therefore,  that,  since  of  all  the  celestial  phenomena 
they  are  indeed  the  most  wonderful,  and  especially 
calculated  to  excite  in  the  minds  of  the  ignorant 
the  idea  of  supernatural  agency,  they  have  been 
universally  regarded  with  the  greatest  apprehension. 


FEARS   EXCITED   BY   COMETS.          17 

Moreover,  when  we  consider  the  wonderful  charac- 
teristics which  mark  their  flight  through  space,  the 
suddenness  and  frequency  of  their  appearance,  and 
the  incongruity  of  their  motion  towards  the  sun 
from  all  regions  and  in  all  directions,  it  will  not 
perhaps  seem  strange  that,  in  ages  of  ignorance  and 
superstition,  they  have  been  regarded  as  the  demon- 
strations of  the  wrath  and  as  the  harbingers  of  the 
vengeance  of  offended  deities  ;  and  that,  among  the 
dreadful  evils  which  they  were  supposed  to  forerun, 
were  pestilence,  famine,  war,  and  political  and 
physical  convulsions. 

It  has  been  well  remarked,  that  it  is  an  inherent 
attribute  of  the  human  mind  to  experience  fear,  and 
not  hope  or  joy,  at  the  aspect  of  that  which  is  unex- 
pected and  extraordinary.  Thus  it  is  that  the 
strange  form  of  a  large  comet,  its  faint  nebulous 
light,  and  its  unpredicted  appearance  in  the  vault 
of  heaven,  have  in  all  countries  been  almost  inva- 
riably regarded  by  the  people  at  large  as  some  new 
and  formidable  agent  inimical  to  the  existing  state 
of  things.  The  sudden  occurrence  and  short  dura- 
tion of  the  phenomena  lead  to  a  belief  in  some 
equally  rapid  and  portentous  reflection  of  its  agency 
in  terrestrial  affairs  ;  while,  on  account  of  the  almost 
infinite  variety  of  the  phenomena  here  presented, 
it  is  easy  to  find  events  that  may  be  regarded  as  the 
fulfilment  of  the  evil  foretold  by  the  appearance  of 


18  TREATISE   ON    COMETS. 

these  mysterious  bodies.  Accordingly,  we  find,  by 
reference  to  the  early  history  of  nations,  when  their 
philosophy  was  essentially  ideal  and  superstitious, 
every  celestial  phenomenon  which  seemed  to  them 
to  be  extraordinary  and  at  variance  with  the  general 
harmony  of  nature,  was  regarded  as  indicative  of 
some  peculiar  state  of  the  weather,  or  of  some  event 
connected  with  the  characters  and  fortunes  of  indi- 
viduals. In  climates  where  the  sky  was  in  a  state 
of  almost  perpetual  serenity,  the  magnificence  of  the 
starry  heavens  could  not  fail  to  excite  the  wonder 
and  admiration  of  the  inhabitants.  Attracted  thus 
unconsciously  to  mark  the  aspects  of  the  heavenly 
bodies,  they  would,  by  continuous  observation,  soon 
acquire  sufficient  familiarity  with  the  general  ap- 
pearance of  the  celestial  vault,  to  detect  any  unu- 
sual appearance  which  might  there  be  exhibited. 
While  thus  occupied  in  observing  phenomena,  they 
would  necessarily  endeavor  to  seek  out  causes  and 
effects ;  and,  since  it  is  inherent  in  the  human  mind 
to  attempt  to  pry  into  the  future,  it  may  readily  be 
perceived  that  their  methods  of  reasoning  would  be 
vitiated  by  labored  attempts  to  connect  the  aspects 
of  the  heavens  with  terrestrial  events.  Speculating 
further,  and  reasoning  by  analogy,  they  would 
necessarily  ascribe  motion  to  the  impulse  of  a  living 
being,  and  as  the  simplest  solution  of  every  such 
appearance,  they  would  suppose  life  to  be  inherent 


OPINIONS   OF   THE   ANCIENTS.         19 

in  the  body  which  moves.  There  is  no  language 
which  does  not  bear  the  traces  of  this  belief.  Every 
fountain  and  every  stream  is  at  first  conceived  to  be 
"living  water,"  every  motion  to  be  the  breathing 
of  a  spirit,  and  the  clouds  floating  on  high  to  be 
borne  along  on  "the  wings  of  the  wind."  In  this 
way,  therefore,  they  would  finally  arrive  at  the  con- 
clusion, whether  legitimate  or  not,  that  the  heavenly 
bodies  which  were  found  to  be  in  motion,  must  be 
in  a  state  of  animation  ;  and  hence  it  would  be  easy 
to  imagine  that  their  power  must  be  incomparably 
superior  to  any  that  could  be  produced  by  human 
agency.  When  such  an  idea  gains  admission,  the 
imagination  knows  not  where  to  stop ;  and  it  has 
been  asserted,  as  a  natural  sequence,  that  this  ideal 
and  visionary  philosophy  would  go  still  further,  and 
maintain  that  sympathies  are  established  between 
the  most  remote  parts  of  the  universe.  All  the  divi- 
sions of  nature  would  be  found  to  harmonize  and 
reciprocally  actuate  each  other.  The  stars  may 
affect  the  earth,  but  the  earth  also  affects  the  stars ; 
and  hence  it  would  be  contended,  that  it  is  possible 
to  acquire  such  a  knowledge  of  their  mutual  actions, 
as  not  only  to  predict  the  changes  which  the  former 
may  produce  in  the  latter,  but,  to  a  certain  extent, 
to  regulate  the  operations  of  the  cause  and  to  modify 
the  degree  of  the  effect. 

Such  we  conceive  to  be  the  primitive  tendency 


20  TREATISE   ON    COMETS. 

of  the  mind  of  man  in  the  early  and  barbarous  state 
of  society,  in  the  study  of  astronomical  science ;  but 
in  more  advanced  stages  of  civilization,  the  opinion 
that  the  heavenly  bodies  have  an  influence  over 
terrestrial  affairs,  might  appear  to  be  confirmed  by 
observations  and  inductions  which  are  not  essentially 
of  so  visionary  a  character.  Certain  appearances  in 
the  heavens  being  associated  with  corresponding 
changes  in  the  seasons,  might  be  supposed  to  be 
either  the  efficient  causes  or  the  invariable  signs  of 
these  changes.  The  variations  in  the  temperature 
and  density  of  the  atmosphere,  the  ebbing  and 
flowing  of  the  tides  of  the  sea,  and  the  fertility  of 
the  earth,  were  known  to  bear  some  intimate  rela- 
tion to  the  sun  and  moon ;  and,  consequently,  it  was 
very  naturally  concluded  that  the  planets  and  the 
comets  must  have  as  great  an  influence  over  the 
bodies  and  minds,  the  actions  and  fortunes  of  men, 
as  those  more  conspicuous  luminaries  have  over  the 
vast  realms  of  the  ocean,  the  air,  and  the  earth. 
The  myriads  of  stars  which  were  seen  scattered 
here  and  there  in  bright  profusion  over  the  entire, 
surface  of  the  heavens,  would,  on  account  of  their 
great  number,  be  supposed  to  have  been  appointed 
to  regulate  the  destinies  of  the  numberless  indi- 
viduals who  inhabit  the  earth,  to  each  of  whom  a 
particular  star  was  appointed,  as  the  guide  of  his 
conduct  and  as  the  arbiter  of  his  fate ;  and,  since 


OPINIONS   OF   THE   ANCIENTS.          21 

many  of  the  stars  were  not  observed  to  have  any 
apparent  connection  with  the  great  changes  which 
have  from  time  to  time  taken  place  in  human  affairs, 
it  would  be  supposed  that  it  was  their  exclusive 
province  to  preside  over  the  incidents  which  occur 
in  the  minuter  portions  of  the  world.  But  the 
comets,  from  their  sudden  and  yet  casual  appear- 
ance, and  their  hideous  aspect,  would  be  regarded 
as  announcing  those  greater  convulsions  which  have 
disturbed  the  nations  of  the  earth.  This  opinion 
would  be  strengthened  by  the  fact  that  it  was  gene- 
rally admitted  that  there  existed  some  intimate 
relation  between  mind  and  matter,  and  that  the 
state  of  the  human  body  and  that  of  the  mind  are 
very  closely  connected.  Now,  since  the  bodily  con- 
stitution is  sensibly  affected  by  the  modifications  of 
the  atmosphere,  it  was  very  naturally  concluded  that 
the  heavenly  bodies  might,  through  the  medium  of 
the  atmosphere,  affect  the  human  body,  and  also, 
through  the  intervention  of  the  body,  affect  the  dis- 
position and  passions  of  the  mind.  It  is  well  known 
that  climate  has  a  most  powerful  agency  in  the  for- 
mation of  human  character,  and  consequently,  if  a 
variation  of  a  few  degrees  in  the  mean  temperature 
of  any  locality  is  sufficient  to  account  for  the  different 
varieties  of  intellectual  capacity,  for  the  strength  or 
weakness  of  passion,  for  the  liveliness  or  defect  of 
imagination,  and  for  the  activity  or  torpor  of  all  the 


22  TREATISE   ox    COMETS. 

faculties,  it  would  not  be  unreasonable  to  suppose 
that  these  varieties  could  be  ascribed  to  influences 
from  the  celestial  regions.  Such  a  conception  seems 
to  harmonize  completely  with  the  doctrines  advanced 
by  some  of  the  ancient  philosophers,  of  the  exist- 
ence of  two  separate  and  distinct  kinds  of  influence, 
—  the  one  immediate  and  the  other  remote,  the  one 
discoverable  by  the  senses,  the  other  eluding  the 
most  careful  observation. 

We  have  thus  discussed,  somewhat  in  detail,  what 
seems  to  be  the  only  reasonable  manner  of  account- 
ing for  the  superstitious  notions  connected  with 
unusual  celestial  appearances,  which  have  prevailed 
in  every  age  of  the  world  and  in  every  nation.  We 
have  been  specific  in  the  statement  of  these  tenden- 
cies of  the  mind,  in  order  that,  although  we  may 
be  amused  at  the  recital  of  the  follies  and  absurd- 
ities of  those  who  fostered  such  delusive  supersti- 
tions, we  may  still  look  beyond  the  immediate  pic- 
ture presented,  and  in  the  dim  vista,  it  may  be, 
witness  the  development  of  each  peculiar  system 
from  what  might  be  considered  perfectly  legitimate 
assumptions.  With  these  preliminary  remarks,  we 
are  prepared  to  consider  the  various  opinions  in 
relation  to  the  appearances  of  comets,  which  have 
been  entertained  in  each  successive  age  of  the 
world,  commencing  with  that  epoch,  where  the 
obscurity  of  tradition  is  dissipated  by  the  light  of 


OPINIONS   OF   THE   ANCIENTS.         23 

authentic  history,  and  tracing  them  forward,  step 
by  step,  until  the  present  day. 

The  ancients  for  the  most  part  regarded  comets 
as  simple  meteors,  generated  by  inflammable  vapors 
in  the  earth's  atmosphere ;  and  supposed  that  when 
they  were  once  extinguished,  they  were  lost  for  ever. 
Some,  however,  believed  them  to  be  distinct  celes- 
tial bodies,  situated  beyond  the  moon.  Diodorus 
informs  us  that  the  shepherd  astronomers  of  Chaldea 
considered  the  comets  as  subject  to  the  same  dynam- 
ical laws  as  the  planetary  bodies,  but  revolving  in 
orbits  which  receded  to  a  greater  distance  from  the 
earth.  The  ancient  Chinese,  unlike  the  Chaldeans, 
were  satisfied  with  simply  extended  observations  of 
comets,  and  did  not  attempt,  even  in  the  slightest 
degree,  the  process  of  generalization  and  of  deriving 
any  knowledge  in  regard  to  their  nature  and  phys- 
ical peculiarities. 

Epigenes,  who  had  studied  among  the  Chaldeans, 
very  singularly  maintained  that  there  were  two 
kinds  of  comets,  the  one  stationary  and  pouring 
forth  their  heat  in  all  directions,  the  other  diffused 
like  hair  and  traversing  the  stars  like  the  planets. 
There  were  many  also  who  believed  that  comets 
were  formed  by  a  conjunction  of  two  planets,  the 
light  of  both  being  confused  into  one,  exhibiting 
the  phenomenon  of  an  elongated  star ;  or,  that  being- 
very  near  each  other,  the  atmosphere  (which  they 


24  TREATISE   ON    COMETS. 

supposed  to  pervade  all  space)  was  enlightened  by 
both,  producing  the  appearance  of  a  comet.  Apol- 
lonius  Myndius  maintained  that  a  comet,  instead  of 
being  composed  of  one  or  more  planets  or  wandering 
stars  in  conjunction,  was  itself  the  same  as  a  planet, 
although  its  form,  unlike  that  of  the  other  planets, 
was  not  globular,  but  more  extended.  He  did  not, 
however,  suppose  them  to  have  any  regular  course, 
but  to  wander  here  and  there,  at  random,  through- 
out the  entire  heavens.  The  same  opinion*  was 
entertained  by  Zeno  the  Stoic.  He  supposed  that 
the  comets  have  their  regular  courses,  and  that  they 
reappear  after  the  lapse  of  a  very  long  interval  of 
time.  The  majority  of  the  Stoics,  however,  ima- 
gined the  comets  to  be  created  in  the  atmosphere, 
and  accounted  for  their  motion  by  supposing  that 
the  rapid  combustion  which  they  believed  to  take 
place,  impelled  them  forward  in  order  to  obtain  a 
sufficient  supply  of  fresh  air. 

Aristotle,  in  an  attempt  to  explain  the  theory  of 
the  tails  of  comets,  was  led  to  believe  that  there 
existed  some  intimate  relation  between  the  comets 
and  the  Milky  Way.  He  supposed  that  the  myriads 
of  stars  which  compose  this  starry  zone,  give  out  a 
self-luminous,  incandescent  matter.  He  therefore 
regarded  the  nebulous  belt  which  separates  the 
different  portions  of  the  vault  of  heaven  as  a  large 
comet,  the  matter  of  which  it  is  composed  being 


OPINIONS   OF   THE  ANCIENTS.         25 

inqessantly  changing,  by  assimilating  new  particles 
and  giving  off  others.  He  also  regarded  comets  as 
presaging  violent  storms  of  wind  and  rain ;  and,  on 
account  of  their  slow  motion,  declared  that  they 
must  be  of  immense  weight.  The  remark  has 
indeed  been  very  appropriately  made,  that,  since 
Aristotle  exercised  so  great  an  influence  throughout 
the  Middle  Ages,  it  is  very  much  to  be  regretted 
that  he  was  so  averse  to  those  grander  views  of  the 
elder  Pythagoreans,  which  inculcated  ideas  so  nearly 
approximating  to  truth  respecting  the  structure  of 
the  Universe.  He  asserts  that  comets  are  transitory 
meteors,  belonging  to  our  atmosphere,  in  the  very 
book  in  which  he  cites  the  opinion  of  the  Pytha- 
gorean school,  according  to  which  these  cosmical 
bodies  are  supposed  to  be  planets,  having  long 
periods  of  revolution.  Panetius  supposed  the  comets 
to  be  delusive  appearances,  produced  by  certain  un- 
explained and  indeterminate  conditions  of  the  atmo- 
sphere ;  and  denied  that  they  exhibited,  either  in 
their  motions  or  appearances,  any  of  the  attributes 
of  material  bodies. 

Such  are  some  of  the  many  curious  theories  which 
were  advanced  by  the  ancient  philosophers,  in  order 
to  account  for  the  phenomena  presented  by  the 
comets.  We  must  not,  however,  omit  to  give  the 
opinions  held  by  Seneca,  who,  among  the  Greeks 
and  Romans,  stands  preeminent  for  his  very  accu- 
3 


26  T  E  E  A  T  I  S  E     0  N     C  0  M  E  T  S  . 

rate  notions  in  regard  to  the  real  character  of  these 
mysterious  bodies.  In  speaking  of  the  various 
theories  which  prevailed  at  his  day,  he  says  :  "  I  do 
not  agree  with  our  philosophers,  for  I  do  not  think 
that  a  comet  is  a  sudden  fire,  but  that  it  is  to  be 
regarded  among  the  eternal  works  of  nature  ;  "  and 
afterwards,  in  recapitulating  his  own  theories,  he 
adds :  "  The  time  will  come  when  some  one  shall 
show  us  in  what  regions  the  comets  wander,  why 
they  are  so  different  from  the  other  celestial  bodies, 
and  what  and  how  great  they  are.  Let  us,  there- 
fore, be  content  with  what  we  know,  and  leave  it  to 
those  who  shall  succeed  us  to  discover  facts  which 
our  day  and  generation  will  not  be  able  to  unfold." 
The  arguments  by  which  Seneca  enforced  his  own 
individual  opinions,  and  the  variety  of  the  pheno- 
mena which  he  cited  in  support  of  each  hypothesis, 
were  wholly  insufficient,  as  we  shall  see,  to  dissem- 
inate and  perpetuate  correct  ideas  of  the  character 
of  the  cometary  worlds.  The  philosophy  of  those 
who  succeeded  him  was  so  impregnated  with  a  kind 
of  religious  superstition,  that  no  argument,  however 
plausible  or  cogent,  seemed  sufficient  to  disabuse 
the  minds  of  the  people  of  those  ideas  of  fear  which 
the  appearance  of  a  comet  invariably  excited.  The 
tendency  of  the  human  mind  to  connect  terrestrial 
events  with  celestial  phenomena,  which  we  have 
already  discussed  somewhat  in  detail,  together  with 


PEARS   EXCITED   BY   COMETS.          27 

the  ignorance  and  stupidity  with  which  a  bigoted 
and  infatuated  priesthood  announced  the  most 
ahsurd  and  ridiculous  doctrines,  with  the  mistaken 
prestige  of  Divine  authority,  had  prepared  the 
people  for  the  adoption  of  the  most  ludicrous  and 
sup'erstitious  fears.  Thus,  while  we  are  prepared  to 
excuse  the  folly  of  less  enlightened  nations,  we 
cannot  avoid  contemplating,  with  mingled  feelings 
of  ridicule  and  amusement,  the  devices  employed 
even  at  a  late  day,  by  nations  professing  to  have 
arrived  at  an  advanced  stage  of  civilization,  in  order 
to  frighten  away  comets  and  counteract  their  sup- 
posed pernicious  and  baneful  influences.  Although 
the  recital  of  the  fears  produced  by  comets  in  vari- 
ous ages  of  the  world,  and  of  the  concurrent  events 
which  they  were  supposed  to  presage,  may  not  be 
considered  essentially  necessary  to  an  understanding 
of  the  true  theory  of  the  influence  of  these  bodies, 
yet  we  deem  it  proper  to  give  a  few  instances  at 
least,  in  order  that,  separated  as  we  are  from  the 
influences  which  operated  in  those  times,  we  may 
regard  them  in  all  their  unmasked  absurdity.  We 
shall  therefore  proceed  at  once  to  give  some  ex- 
amples. 

We  are  informed  by  Seneca,  on  the  authority  of 
Calisthenes,  that  immediately  prior  to  the  great 
earthquake  in  Achaia,  in  the  year  373  B.  C.,  there 
appeared  a  comet  of  enormous  size,  which  was  the 


28  TREATISE   ON   COMETS. 

specific  cause  of  the  destruction  of  the  cities  of 
Bura  and  Helice.  Those  cities  were  submerged  by 
the  sea,  and  the  appearance  of  the  comet  at  the 
same  epoch,  and  its  disappearance  very  soon  after 
that  event,  led  to  a  very  general  belief  that  its  office 
was  directly  connected  with  the  destruction  of  both 
cities.  The  earthquake  here  referred  to  was,  per- 
haps, the  greatest  and  most  disastrous  which  had 
ever  been  felt  in  that  portion  of  the  globe.  It  is 
not  strange,  therefore,  that  Aristotle  should  have 
shared  the  general  belief;  and,  accordingly,  we  find 
that  he  makes  mention  of  it  as  presaging  the  dread- 
ful events  which  took  place  in  the  same  year  in 
which  it  appeared.  He  informs  us,  also,  that  in  the 
year  468  B.  C.,  there  appeared  a  large  comet,  which 
was  supposed  to  have  some  agency  in  producing  the 
famous  fall  of  aerolites  near  ^Egos  Potamos.  One 
of  these  meteoric  stones,  celebrated  in  antiquity,  is 
said  to  have  weighed  nearly  two  tons.  It  was  seen 
to  fall  from  an  immense  cloud  of  fire  and  smoke ; 
and  the  occurrence  of  such  an  unusual  event,  to- 
gether with  the  phenomena  presented  by  the  cornet, 
produced  a  most  profound  sensation  among  all 
classes.  Aristotle  mentions  another  comet  which 
appeared  in  the  year  341  B.  C.,  which  was  supposed 
to  have  caused  a  terrific  storm  near  Corinth. 

In  the  year  43  B.  C.  a  comet  made  its  appearance 
which  was  so  brilliant  as  to  have  been  visible  to  the 


FEARS   EXCITED   BY   COMETS.          29 

naked  eye  in  the  daytime,  when  in  the  immediate 
vicinity  of  the  sun.  It  was  regarded  by  the  Romans 
as  the  soul  of  Julius  Caesar  —  who  had  lately  been 
murdered  —  transferred  to  the  heavens.  The  star 
of  the  Magi,  which  signalized  the  birth  of  Christ, 
is  numbered  among  the  comets  by  many  modern 
writers  on  the  subject.  The  probability  of  the  iden- 
tity of  the  star  of  the  Magi  and  a  comet  which 
appeared  the  same  year,  has  been  doubted.  The 
comet  is  described  as  producing  a  light  rivalling  the 
splendor  of  the  noonday  sun.  It  is  said  to  have 
appeared  during  an  interval  of  twenty-four  days, 
and  its  magnitude  was  so  great  that  it  extended 
over  nearly  fifty  degrees  of  the  heavens,  so  as  to 
occupy  more  than  three  hours  in  rising  and  setting. 
A  comet  appeared  in  the  year  54  of  our  era,  which 
was  supposed  to  have  presaged  the  death  of  the 
Emperor  Claudius,  which  took  place  on  the  13th  of 
October  of  the  same  year ;  and  a  comet  which  ap- 
peared in  the  year  63  was  supposed  to  have  been 
the  cause  of  earthquakes  in  Achaia  and  Macedonia. 
Josephus  informs  us.  that  just  before  the  destruction 
of  Jerusalem  by  Titus  Vespasian,  in  the  year  69, 
there  appeared,  hanging  over  the  city,  a  flaming 
torch;  and  ten  years  later,  while  Vespasian  was 
suffering  from  the  disease  of  which  he  died,  a  comet 
was  seen,  which  continued  in  sight  during  a  long 
time.  In  the  latter  part  of  the  year  190,  or  in  the 
3* 


30  TREATISE   ON    COMETS. 

beginning  of  191,  a  "hairy  star"  was  seen,  which 
Herodian  informs  us  was  the  cause  of  many  prodi- 
gies which  appeared  at  the  same  time.  Stars  were 
continually  seen  by  daylight,  and  some  of  them  ap- 
peared to  be  stretched  out  lengthways,  and  seemed 
to  be  suspended  in  the  air. 

The  death  of  the  Emperor  Constantine  is  said  to 
have  been  preceded  for  several  days  by  the  appear- 
ance of  a  comet  of  extraordinary  magnitude,  and 
another  comet  which  appeared  in  the  year  400  was 
believed  to  have  foreboded  the  most  frightful  dis- 
asters. We  are  informed  by  the  historians  of  the 
Western  Empire  that  the  misfortunes  with  which 
Constantinople  was  threatened  by  Gain  as,  in  the 
same  year,  were  so  great  that  they  were  undoubt- 
edly announced  by  the  appearance  of  the  comet, 
which  is  described  as  being  one  of  the  most  terrible 
ones  on  record.  It  appeared  above  the  city,  with  a 
tail  in  the  form  of  a  sabre,  extending  from  the  high- 
est region  of  the  sky  almost  to  the  horizon.  Two 
years  later  another  comet  made  its  appearance, 
which  seemed  fairly  to  rival  this  one.  It  was  be- 
lieved to  have  caused  tempests  and  frequent  thunder, 
numerous  eclipses  of  the  moon,  prodigious  hail 
storms,  and  spontaneous  conflagrations.  It  was 
said  to  have  been  attended  by  birds  of  evil  augury, 
and  to  have  announced  the  arrival  of  Alaric,  King 


FEARS    EXCITED    BY    COMETS.  31 

of  the  Visigoths,  in  Italy,  an  event  which  caused 
the  greatest  possible  consternation  at  Rome. 

A  comet  which  appeared  immediately  before  the 
birth  of  Mohammed  was  subsequently  supposed,  by 
the  followers  of  the  Prophet,  to  have  announced  his 
birth,  in  a  manner  precisely  similar — except  in  de- 
tail—  to  the  star  which  appeared  at  the  birth  of 
Christ, 

A  comet  appeared  in  March,  1402,  which  was  so 
brilliant  as  to  be  distinctly  visible  at  mid-day ;  and 
a  second  comet,  which  appeared  in  June  of  the  same 
year,  was  visible  several  hours  before  sunset.  This 
comet  was  believed  to  presage  the  death  of  John 
Galeas  Visconti.  That  prince,  being  a  believer  in 
astrology,  had  consulted  the  charlatans  of  his  court 
in  reference  to  such  an  event,  and  the  fright  pro- 
duced by  the  appearance  of  the  comet  may  possibly 
have  contributed  to  the  fulfillment  of  the  prediction 
of  his  death.  Another  conspicuous  comet  appeared 
in  1532,  which  was  also  stated  to  have  been  visible 
before  sunset.  It  created  a  most  profound  sensa- 
tion throughout  France,  Germany,  and  Northern 
Italy,  where  it  was  considered  as  the  announcement 
of  the  death  of  Sforza  II. 

The  celebrated  comet  of  Halley,  which  we  shall 
subsequently  refer  to,  performs  its  revolution  around 
the  sun  in  a  period  of  75  years,  and,  consequently, 
has  been  visible  at  many  different  epochs.  It  may 


32  TREATISE   ON    COMETS. 

not  be  uninteresting,  therefore,  in  order  to  show  the 
discrepancies  of  all  superstitious  theories,  to  give, 
in  a  connected  manner,  all  the  various  events  which 
the  same  body  has  been  supposed  to  announce  at 
different  visits  to  our  immediate  vicinity,  although 
the  fact  of  the  periodicity  of  the  comet  was  unknown 
to  those  who  attributed  to  it  such  remarkable  quali- 
ties. In  1005  the  appearance  of  this  comet  was 
attended  by  a  great  famine ;  in  1080  by  an  earth- 
quake ;  in  1155  by  a  cold  winter  and  the  failure  of 
crops ;  in  1230  by  rain  and  inundations  (part  of 
Friesland  was  overwhelmed,  with  100,000  inhabit- 
ants) ;  in  1304  by  great  drought,  and  intense  cold 
in  the  following  winter,  succeeded  by  a  pestilence ; 
in  1380  by  a  still  more  destructive  contagion ;  and 
in  1456  by  wet  weather,  inundations,  and  earth- 
quakes. At  its  appearance  in  1456  the  comet  is 
represented  as  being  of  an  "  unheard  of  magnitude," 
and  as  having  a  tail  which  extended  over  sixty  de- 
grees of  the  heavens,  or  nearly  from  the  zenith  to 
the  horizon.  It  was  visible  during  the  month  of 
June,  and  spread  terror  throughout  Europe.  It  was 
supposed  to  announce  the  future  success  of  the 
Turks  under  Mohammed  II.,  who  was  then  engaged 
in  the  subjugation  of  the  Christian  nations.  He 
had  already  taken  Constantinople  in  part,  had 
advanced  his  forces  even  as  far  as  the  walls  of  Vi- 
enna, and  had  thus  struck  terror  into  the  whole 


F  E  A  it  ,S     EXCITED     BY     C  0  M  E  T  S  .  33 

Christian  world.  Pope  Calixtus  II.,  terrified  at  the 
appalling  spectacle  presented  by  the  comet,  and 
aroused  by  what  seemed  to  be  the  inevitable  fate 
of  Christianity,  directed  the  thunders  of  the  Church 
against  the  enemies  of  the  faith,  both  terrestrial 
and  celestial.  It  had  become  imperatively  necessary 
that  something  should  be  done  immediately  to 
counteract  the  baneful  influences  which  were  ope- 
rating both  above  and  below ;  and,  consequently,  the 
Pope  ordered  public  prayers,  and  issued  a  bull,  in 
which  he  anathematized  not  only  the  Turks,  but  the 
comet ;  and  in  order  to  perpetuate  this  manifestation 
of  the  power  of  the  Church,  he  ordained  that  the 
bells  should  be  rung  at  noon,  a  custom  which  is 
still  observed  in  various  Catholic  countries.  But, 
as  Pontecoulant  has  aptly  remarked,  neither  the 
progress  of  the  comet,  nor  the  victorious  arms  of 
the  Mohammedans,  were  arrested.  The  comet  tran- 
quilly proceeded  in  its  orbit,  passing  through  its 
appointed  changes,  regardless  of  the  thunders  of 
the  Vatican,  and  the  Turks  under  Mohammed  esta- 
blished their  principal  mosque  in  the  church  of  St. 
Sophia. 

In  the  year  1531  the  appearance  of  this  comet  is 
said  to  have  been  attended  by  great  floods ;  in  1607 
by  extreme  drought,  followed  by  a  most  severe 
winter ;  in  1682  by  floods  and  earthquakes ;  in  1759 
by  rains  and  storms,  and  slight  earthquakes ;  while 


34  TREATISE   ON   COMETS. 

in  1835,  the  date  of  its  last  appearance,  nothing,  so 
far  as  we  are  aware,  has  been  attributed  to  it,  ex- 
cept, it  may  be,  some  local  epidemic  or  unfavorable 
state  of  the  weather. 

It  is  related  that  a  comet  appeared  in  the  year 
590,  to  the  presence  and  influence  of  which  was 
ascribed  a  fearful  epidemic,  which  prevailed  in  that 
year,  in  the  crisis  of  which  the  patients  were  seized 
with  violent  paroxysms  of  sneezing,  often  followed 
by  death.  It  has  been  asserted,  also,  that  it  became 
the  custom,  when  these  paroxysms  manifested 
themselves,  for  the  bystanders  to  address  their 
benediction  to  the  sufferer,  exclaiming,  "  God  bless 
you;"  and  to  this  has  been  attributed  the  custom 
which  is  prevalent  at  this  day,  to  address  one  who 
happens  to  sneeze  with  the  same  words. 

In  a  work  on  the  atmospherical  origin  of  epi- 
demic diseases,  written  by  an  English  physician, 
about  thirty  years  since,  it  is  asserted  that,  since  the 
Christian  era,  the  most  unhealthy  periods  have  been 
precisely  those  in  which  some  great  comet  appeared; 
that  such  appearances  were  accompanied  by  earth- 
quakes, volcanic  eruptions,  and  atmospheric  com- 
motions, while  no  comet  has  been  observed  during 
healthy  periods.  Among  the  many  cometary  influ- 
ences enumerated  are  the  following:  Hot  seasons 
and  cold,  tempests,  earthquakes,  volcanic  eruptions, 
hail,  rain,  and  snow,  floods  and  droughts,  famines, 


FEARS   EXCITED   BY    COMETS.          35 

clouds  of  midges  and  locusts,  the  plague,  dysentery, 
and  influenza.  Each  affliction  is  assigned  to  its 
comet,  whatever  kingdom,  city,  or  village,  the 
famine,  pestilence,  or  other  visitation  may  have 
ravaged.  In  making  thus,  from  year  to  year,  a 
complete  inventory  of  the  misfortunes  of  this  lower 
world,  the  question  has  very  properly  been  asked, 
who  would  not  have  foreseen  the  impossibility  of 
any  comet  approaching  the  earth,  without  finding 
some  portion  of  its  inhabitants  suffering  under  some 
affliction  ;  and  who  would  not  have  granted  at  once, 
what  Lubienietski  has  written  a  large  work  to  prove, 
that  there  never  was  a  disaster  without  a  comet,  nor 
a  comet  without  a  disaster  ?  These  are  only  a  few 
of  the  influences  recorded  in  the  work  above  re- 
ferred to,  but  there  is  one  in  particular  which  must 
be  regarded  as  the  masterpiece  of  absurdity,  namely, 
that  the  appearance  of  the  great  comet  of  1668  pro- 
duced a  remarkable  epidemic  among  the  cats  in 
Westphalia. 

The  great  comet  which  appeared  in  1811  is  said 
to  have  produced  some  of  the  most  remarkable 
results.  Among  others,  it  is  asserted,  in  a  standard 
English  periodical,  published  a  few  years  subsequent 
to  the  appearance  of  the  comet,  that  its  influence 
produced  a  mild  winter,  a  moist  spring,  and  a  cold 
summer;  that  there  was  not  sufficient  sunshine  to 
ripen  the  fruits  of  the  earth ;  that,  nevertheless 


36  TREATISE   ON    COMETS. 

(such  was  the  cometic  influence),  the  harvest  was 
abundant,  and  some  species  of  fruits,  such  as  melons 
and  figs,  were  not  only  plentiful,  but  of  a  delicious 
flavor;  that  wasps  rarely  appeared,  and  flies  became 
blind  and  died  early  in  the  season ;  that  in  the 
neighborhood  of  London,  numerous  instances  oc- 
curred of  women  bearing  twins,  and  it  even  hap- 
pened, in  one  instance,  that  the  wife  of  a  shoemaker 
in  Whitechapel  had  four  children  at  a  birth  !  All 
these  singular  influences  were  assigned  to  this 
comet.  In  Germany  the  somewhat  remarkable 
crop  of  grapes  produced  in  that  year  was  duly 
acknowledged  to  be  the  effect  of  the  comet ;  and 
the  wine  manufactured  therefrom,  which  received 
the  appellation  of  the  comet  wine,  obtained  a  popu- 
larity and  demand  which  has  rarely,  if  ever,  been 
equalled. 

The  celebrated  traveller  Riippel,  in  writing  from 
Cairo  on  the  8th  of  October,  1835,  observes  that  the 
Egyptians  thought,  that  the  comet  then  visible  was 
the  cause  of  the  great  earthquake  which  was  felt  in 
that  country  on  the  21st  of  August  of  that  year,  and 
that  the  comet  also  exercised  so  malignant  an  influ- 
ence over  some  of  the  lower  animals,  that  horses 
and  asses  perished  in  great  numbers.  The  truth 
was,  he  adds,  that  the  poor  animals  died  of  star- 
vation, their  usual  forage  having  failed  in  conse- 
quence of  the  insufficient  inundation  of  the  Nile. 


FEARS   EXCITED   BY   COMETS.          37 

Such  are  a  few  examples  of  the  many  curious  and 
absurd  notions  of  cometary  influences  which  have 
prevailed  at  various  periods  in  the  history  of  the 
world.  We  might  suppose  that  in  these  enlightened 
times,  no  such  attributes  would  be  assigned  to  these 
chaotic  worlds,  especially  in  countries  where  edu- 
cation is  generally  diffused  among  the  people,  and 
where  the  arts  and  sciences  have  been  carried  to  a 
high  state  of  perfection.  We  ought  to  expect  to 
find,  in  these  cases,  that  the  appearance  of  the  comet 
would  be  everywhere  hailed  with  delight,  as  fur- 
nishing new  proofs  of  the  beautiful  harmony  which 
characterizes  the  motions  of  the  heavenly  bodies. 
But  such  is  not  generally  the  case.  The  same  ten- 
dency of  the  human  mind  to  connect  terrestrial 
events  with  celestial  phenomena,  exhibits  itself  even 
when  civilization,  culture,  and  refinement  appear  to 
have  attained  the  foremost  rank.  We  even  find 
that  in  the  days  of  Shakspeare,  the  comets  were  re- 
garded as  being  directly  connected  with  the  great, 
more  especially  the  calamitous  events  of  nations, 
and  are  thus  introduced  by  the  illustrious  poet  him- 
self, in  the  lamentation  which  the  Duke  of  Bedford 
makes  over  the  bier  of  Henry  V. : 

"  Comets,  importing  change  of  times  and  states, 
Brandish  your  crystal  tresses  in  the  sky ; 
And  with  them  scourge  the  bad  revolting  stars, 
That  have  consented  unto  Henry's  death." 

4 


38  TREATISE   ON    COMETS. 

Milton,  too,  though  he  lived  in  the  days  of  Kepler 
and  Galileo,  though  he  was  imbued  with  all  the 
learning  and  philosophy  of  his  time,  and  showed 
that  he  was  intimately  acquainted  with  all  the  labors 
of  the  philosophers  who  had  preceded  or  were 
cotemporaneous  with  him,  does  not  scruple  to  call 
in  the  aid  of  the  malign  power  of  comets,  in  order 
to  heighten  his  picture  of  Satan  when  preparing  for 
the  combat: 

.     "On  the  other  side, 
Incensed  with  indignation,  Satan  stood, 
Unterrified,  and  like  a  comet  burned, 
That  fires  the  length  of  Ophiuchus  huge 
In  the  Arctic  sky,  and  from  its  horrid  hair 
Shakes  pestilence  and  war." 

When  such  men  as  Milton  and  Shakspeare  allude 
to  the  comets  in  such  a  manner,  even  for  the  pur- 
pose of  illustration,  we  will  not  be  surprised  to 
know  that  sentiments  similar  to  these  were  enter- 
tained by  the  people  at  large.  But  to  come  down 
even  to  a  later  day,  to  the  date  of  the  last  appear- 
ance of  Halley's  comet  (1835),  we  shall  find  that 
notions  fully  as  absurd  as  those  which  we  have 
already  noticed,  were  received  in  France  and  else- 
where. But  a  short  time  previous  to  the  approach 
of  the  comet  to  the  earth  and  sun,  Arago  wrote  as 
follows :  "I  would  have  wished,  for  the  honor  of 
modern  philosophy,  to  be  freed  from  the  necessity 
of  taking  serious  notice  of  such  absurdities ;  but  I 


I  N  F  L  U  E  X  C  E     ON     THE     WEATHER.  39 

have  acquired  personal  knowledge  that  some  refu- 
tation of  them  is  not  useless,  and  that  the  advocates 
of  these  influences  have  no  inconsiderable  number 
of  followers.  Listen,  when  you  are  present  at  one 
of  those  brilliant  assemblies,  where  you  meet  what 
is  called  good  society ;  listen  to  the  talk,  of  which 
the  approaching  comet  furnishes  the  subject,  and 
then  decide  if  we  ought  to  boast  of  that  diffusion 
of  knowledge,  which  so  many  declare  to  be  the 
characteristic  feature  of  our  times." 

Those,  however,  who  assign  to  the  comets,  not  an 
influence  over  the  fortunes  of  nations  and  indi- 
viduals, directly,  but  rather  an  influence  over  atmo- 
spheric changes,  may  perhaps  do  so  with  a  greater 
show  of  probability.  To  maintain  that  the  comets, 
owing  to  their  peculiar  nature,  may,  to  a  certain  lim- 
ited extent,  operate  to  produce  some  unusual  state  of 
the  weather,  will  not  at  first  be  regarded  by  a  large 
portion  of  even  intelligent  people,  as  really  absurd. 
There  are  numerous  reasons,  and  even  forcible  ones, 
in  theory,  which  may  be  adduced  in  support  of  such 
a  hypothesis;  but  there  are  other  considerations 
which  are  of  far  greater  importance,  and  have  a 
bearing  in  the  opposite  direction,  which  must  not 
be  overlooked.  It  would,  indeed,  be  easy  to  show 
at  once,  upon  general  physical  principles,  that  there 
is  no  reason  whatever  why  a  comet  should  exert 
any  influence,  even  in  the  very  slightest  degree, 


40  TREATISE   ON   COMETS. 

over  the  temperature  of  our  seasons,  or  in  producing 
either  droughts  or  rains ;  but  it  will  perhaps  be 
more  satisfactory  to  refute  it  by  showing  that  such 
doctrines  are  not  in  conformity  with  observed  facts. 
This  method  of  showing  the  utter  fallacy  of  attri- 
buting to  comets  any  direct  influence  other  than 
what  is  due  to  their  infinitesimal  attractive  influence 
in  accordance  with  the  law  of  universal  gravitation, 
is  most  simple  and  easy.  The  appearances  and 
motions  of  the  comets  have  been  recorded.  The 
average  daily,  monthly,  and  yearly  temperatures  of 
the  weather,  the  direction  and  velocity  of  the  winds, 
the  number  and  character  of  the  storms,  and  the 
extent  of  the  droughts,  have  also  been  accurately 
observed  and  recorded.  To  ascertain,  therefore, 
whether  the  comets  really  exercised  any  influence 
on  the  temperature  of  the  seasons,  it  is  only  neces- 
sary to  place  in  juxtaposition  these  two  records,  and 
to  examine  whether  there  exists  any  general  law  or 
analogy  which  is  capable  of  exhibiting  any  corre- 
spondence between  them. 

Arago  has  given  a  table,  in  which  he  has  exhi- 
bited in  one  column  the  temperatures  of  the  weather 
at  Paris  for  every  year,  from  1735  to  1831,  inclusive ; 
and  in  juxtaposition  with  these  he  has  stated  the 
number  of  comets  which  appeared,  with  their  mag- 
nitude and  general  appearance.  The  result  is  that 
no  coincidence  whatever  is  observable  between  the 


INFLUENCE   ox   THE   WEATHER.        41 

temperatures  and  the  number  and  appearance  of 
comets.  Sometimes  we  find  that  the  years  of  great- 
est mean  temperature  were  those  in  which  several 
comets  appeared ;  and,  again,  that  they  were  those 
in  which  no  comet  appeared.  For  example,  in 
1737,  although  two  comets  appeared,  the  mean  tem- 
perature was  inferior  to  that  of  the  preceding  years, 
in  which  none  appeared.  The  year  1765,  in  which 
no  comet  appeared,  was  hotter  than  the  year  1766, 
when  two  comets  appeared,  one  of  which  was  re- 
markable for  its  splendor ;  the  year  1775,  when  no 
comet  appeared,  was  hotter  than  the  year  1780, 
which  was  marked  by  the  appearance  of  two  comets. 
The  temperature  was  still  lower  in  1785,  in  which 
two  comets  appeared ;  while,  on  the  other  hand,  the 
temperature  of  the  year  1781  was  greater,  which 
was  likewise  marked  by  the  appearance  of  two 
comets. 

This  question  of  the  supposed  connection  between 
the  temperature  and  the  appearance  of  comets,  was 
still  farther  discussed  by  Arago.  He  has  given  not 
only  the  general  temperatures,  but  also  a  table  of 
the  years  of  greatest  cold,  of  the  years  in  which  the 
Seine  has  been  frozen  over,  and  also  of  the  years 
of  greatest  heat ;  and  he  has  shown  that  the  corre- 
sponding appearances  of  comets  have  been  varied 
without  any  connection  whatever  with  these  vicis- 
situdes of  temperature.  Thus,  1806  and  1811  were 
4* 


42  TREATISE   ON    COMETS. 

both  hot  years  —  the  first,  however,  hotter  than  the 
second ;  and  yet  the  first  had  only  one  comet,  while 
the  second  had  two,  one  of  which  was  the  most 
hrilliant,  with  perhaps  a  single  exception,  which  has 
appeared  during  the  present  century.  Again,  the 
year  1826,  in  which  five  comets  appeared,  was  not 
so  hot  as  1831,  which  was  characterized  by  the 
appearance  of  only  one  comet.  But  it  is,  perhaps, 
unnecessary  to  enter  into  any  further  detail  in  regard 
to  the  thermal  character  of  particular  seasons  as 
compared  with  cometary  appearances,  and  we  may 
therefore  notice  some  of  the  more  prominent  results 
arrived  at  by  Arago. 

Of  seventy-four  years,  forty-nine  were  signalized 
by  the  appearance  of  one  or  more  comets,  and 
twenty-five  by  their  non-appearance.  The  mean 
temperature  of  the  former  years  was  found  to  be 
51.°6,  and  that  of  the  latter  50.°7,  the  difference,  as 
will  be  perceived,  being  actually  less  than  a  degree. 
Again,  of  the  forty-nine  years  in  which  comets  ap- 
peared, a  single  one  was  observed  in  twenty-five  of 
these  years,  and  two  or  more  in  the  remaining 
twenty-four  years.  Now,  if  these  bodies  produced 
any  influence  on  the  temperature,  it  might  naturally 
be  expected,  or  rather  it  would  be  a  necessary  con- 
sequence, that  a  difference  would  be  found  to  exist 
between  the  mean  temperature  of  the  latter  and  of 
the  former  years.  It  was  found,  however,  that  the 


INFLUENCE  ON  THE  WEATHER.   43 

mean  temperature  of  the  twenty-five  years  in  each 
of  which  a  single  comet  appeared,  was  51.  °6,  while 
that  of  the  twenty-four  years  in  which  there  were 
several  comets,  was  51. °4;  the  difference  being  only 
the  fifth  part  of  a  degree,  and  even  that  being  in  a 
direction  contrary  to  the  hypothesis  that  the  comets 
tend  to  augment  rather  than  to  diminish  the  tern- 
perature.  It  is  noticeable,  however,  in  these  com- 
parisons, that  as  a  general  result,  more  comets  are 
seen  in  hot  years  than  in  cold  ones,  which  may  be 
explained  by  the  fact  that  the  hot  years,  generally 
giving  a  finer  sky,  are  more  favorable  than  the 
colder  ones  for  the  discovery  of  those  objects  which 
are  usually  too  faint  to  be  detected  by  the  naked 
eye.  In  the  case  of  the  large  comets  the  circum- 
stance just  noticed  would  not  produce  any  effect, 
and  accordingly  we  find  that  in  the  year  1843,  the 
date  of  the  appearance  of  one  of  the  most  magni- 
ficent comets  on  record,  the  average  temperature 
was  more  than  a  degree  less  than  it  had  been  for 
several  years  previous.  Again,  the  mean  tempera- 
ture of  the  years  1853  and  1858,  in  each  of  which 
there  appeared  large  and  brilliant  comets,  did  not 
sensibly  differ  from  the  average  temperature  of  a 
series  of  years.  Another  fact  which  might  be  no- 
ticed in  this  connection,  is  that  experiment  showed, 
in  the  case  of  the  great  comet  of  1811,  that  the 
light  emitted  by  the  comet,  when  brought  to  a  focus 


44  TREATISE   ON   COMETS. 

by  a  large  mirror,  was  not  equal  to  a  tenth  part  of 
the  light  of  the  full  moon.  Moreover,  it  was  found 
to  have  no  sensible  effect  on  the  blackened  ball  of 
a  thermometer,  which  was  so  delicate  a  test  of  vari- 
ation of  temperature  that  it  would  have  indicated 
the  hundredth  part  of  a  degree.  This  alone  shows 
the  total  inefficiency  of  comets  to  influence  our  cli- 
mate even  in  the  slightest  degree.  In  fine,  the  com- 
plete discussion  of  the  cometary  and  thermal  obser- 
vations, continued  through  the  entire  period  during 
which  authentic  records  of  each  have  been  kept, 
fully  establishes  the  conclusion  that  there  exists  no 
foundation  whatever  for  the  popular  opinion  that 
the  comets  influence  the  seasons. 

There  is  another  point  of  view  in  which  the  sup- 
posed agency  of  comets  in  producing  atmospherical 
changes  has  been  presented,  which  requires  reason- 
ing of  quite  a  different  character  from  that  which 
we  have  already  given.  It  is  imagined  that  the 
comet  has  a  certain  magnetic  influence,  of  which 
the  law  of  intensity  and  of  operation  is  unknown. 
For  example,  it  is  now  universally  admitted  that 
there  exists  some  direct  connection  between  terres- 
trial magnetism  and  the  solar  spots,  and  in  the 
same  way  it  is  conceived  that  there  may  exist  also 
a  direct  connection  between  the  comets  and  our 
atmosphere.  Those  who  have  advanced  such  theo- 
ries have  generally  adopted  Oersted's  hypothesis. 


INFLUENCE   ON   THE   WEATHER.        45 

that  light,  heat,  and  electricity  are  produced  in  the 
same  manner,  yet  under  different  circumstances; 
and  that  it  is  the  ethereal  fluid  which  serves  for  the 
transmission  of  light  by  successive  undulations, 
which  furnishes  the  agent  for  heat  and  electricity. 
They  conceive,  therefore,  that  the  comet  is  in  a  state 
of  electrical  excitement,  and  that  it  operates  by  in- 
duction through  the  ether,  and  affects  the  atmo- 
sphere. This  is  a  general  statement  of  the  theory, 
although  its  development  by  different  individuals 
has  led  to  a  great  variety  of  absurd,  and,  for  the 
most  part,  conflicting  results.  The  best,  and  per- 
haps *the  only'  argument  necessary  to  refute  these 
doctrines,  is  the  fact  that  observation  fails  to  exhibit 
any  agreement  or  harmony  in  the  phenomena  which 
could  be  reconciled  with  any  recognized  or  supposed 
law  in  physics.  It  fails  in  every  instance  to  exhibit 
the  least  connection  between  atmospherical  changes 
and  cometary  appearances ;  and  in  view  of  all  this, 
we  cannot  conclude  otherwise  than  that  the  appear- 
ance of  a  comet  is  no  more  a  prodigy,  and  has  no 
more  influence  upon  the  fate  of  men  or  of  nations, 
than  the  appearance  of  the  moon  or  of  any  planet 
or  star  in  the  firmament.  In  reality  it  cannot  have 
nearly  so  much  ;  for  the  moon,  by  causing  the  tides, 
affects  both  the  atmosphere  and  the  weather,  and 
thus  indirectly  it  affects  also  the  human  body.  The 
comets,  on  the  contrary,  are  so  distant,  and  either 


46  TREATISE   ON    COMETS. 

their  motions  are  so  rapid  or  their  substance  so  rare, 
that  none  of  them  have  been  found  to  have  any 
material  action  on  such  of  the  planets  as  they  may 
have  approached  very  near,  although  the  planets 
have  been  found  to  have  a  very  considerable  influ- 
ence on  them. 

"We  have  thus  shown  conclusively  that  the  general 
opinion  which  prevails,  even  at  the  present  day, 
among  all  classes  of  people,  that  the  appearance  of 
a  comet  has  reference  to,  or  rather  causes,  some 
unusual  state  of  the  weather,  is  without  the  slightest 
foundation,  either  in  philosophy  or  in  fact.  The 
arguments  are  such  as  to  admit  of  no  equivocation, 
and  every  intelligent  reader  will  at  once  perceive 
how  untenable  and  unworthy  of  confidence  the 
whole  theory  of  cometary  influence  in  terrestrial 
affairs  must  be.  There  are,  however,  two  other 
cases  connected  with  this  branch  of  our  subject, 
which  must  not  be  passed  by  unnoticed,  and  which 
we  shall  now  consider,  namely,  the  possibility  of  the 
near  approach  of  a  comet  so  as  to  produce  atmo- 
spheric disturbances  and  currents,  and  also,  the 
possibility  of  a  collision  between  a  comet  and  the 
earth,  and  the  effect  of  such  a  collision. 

We  shall  hereafter  have  occasion  to  show  that 
some  of  the  comets  have  trains  which  extend  to 
such  an  enormous  distance,  that  were  the  earth  in 
the  line  of  the  axis  of  the  tail  it  would  certainly  be 


NEAR  APPROACH  TO  THE  EARTH.  47 

enveloped  in  it,  since  the  length  of  the  tail  often 
exceeds  the  distance  of  the  comet  from  the  earth. 
Should  it  happen,  therefore,  that  on  the  approach 
of  a  very  large  cornet  to  its  perihelion,  or  point 
nearest  the  sun,  the  earth  should  have  a  position 
such  that  the  sun  and  comet  would  appear  to  be  in 
conjunction,  or  nearly  so,  the  inevitable  result  must 
be  that  the  earth  would  be  surrounded  completely 
by  the  nebulous  matter  which  forms  the  tail  of  the 
comet,  but  which  we  shall  find  to  exist  in  a  state  of 
great  attenuation.  Thus,  although  the  possibility 
of  such  an  event  is  fairly  established,  yet,  if  we  rea- 
son from  the  theory  of  probabilities,  and  introduce 
the  comets  which  have  hitherto  been  observed  as 
the  basis  of  our  computations,  we  shall  find  that  the 
probability  of  an  approach  sufficiently  near  to  en- 
velop the  earth  in  the  tail,  will  not  exceed  the  ratio 
of  one  to  one  hundred  millions.  In  other  words, 
let  us  suppose  the  number  of  comets  which  shall 
pass  through  our  system  in  a  given  time,  and  whose 
perihelia  are  within  the  orbit  of  the  earth,  to  be  one 
hundred  millions ;  then  it  is  barely  possible,  or 
rather  probable,  that  one  of  these  may  envelop  the 
earth  in  its  tail.  When  such  an  event  will  take 
place,  whether  it  ever  has  taken  place,  or  whether 
it  ever  will  occur,  is  a  problem  which  defies  the 
power  of  human  analysis ;  but  since  the  possibility 


48  TREATISE   ON    COMETS. 

cannot  be  doubted,  it  will  not  be  improper  to  con- 
sider what  would  be  the  probable  effect  produced. 

It  is  well  known,  that  since  the  comets  are  found 
to  be  composed  of  matter  extremely  attenuated,  the 
tails  will,  as  a  necessary  consequence,  be  even  more 
attenuated ;  and  in  case  the  earth  should  be  enve- 
loped in  one  of  these,  it  would  hardly  be  expected 
that  the  nebulous  matter  would  even  penetrate  our 
atmosphere  to  any  considerable  distance.  There 
might,  however,  exist  such  conditions,  as  regards 
the  nebulosity  of  the  tail  and  the  atmosphere  of  the 
earth,  as  would  result  in  the  complete  permeation 
of  the  atmosphere  by  the  matter  of  the  tail.  As  a 
general  thing,  no  such  result  need  be  feared,  in  case 
the  circumstances  were  such  as  to  render  it  possible  ; 
and  even  if  it  should  really  happen,  the  effect  pro- 
duced might  be  most  beneficial  to  mankind.  The 
nebulous  matter  might  be  of  such  a  character  as  to 
cause  the  most  fatal  epidemics ;  or,  on  account  of 
atmospheric  disturbances  and  currents  which  it 
would  generate,  it  might,  on  the  contrary,  be  such 
as  to  produce  a  sanitary  effect.  That  atmospheric 
currents  exist  in  certain  parts  of  the  globe,  which 
do  exercise  a  healing  influence  over  various  kinds 
of  diseases,  has  been  certainly  established.  For 
example,  the  influence  of  the  Harmattan,  a  peri- 
odical wind,  which  blows  three  or  four  times  a  year 
from  the  interior  of  the  African  continent  towards 


THEHARMATTAN.  49 

the  Atlantic  coast,  between  the  latitudes  fifteen 
degrees  north  and  one  degree  south,  is  thus  de- 
scribed by  an  English  traveller: 

"  The  periods  of  its  prevalence  are  stated  to  be 
chiefly  from  the  end  of  November  to  the  beginning 
of  April,  its  direction  varying  from  east-south-east 
to  north-north-east.  Its  duration  at  any  one  time 
varies  from  one  to  six  days,  and  its  force  is  always 
very  moderate.  A  fog,  thick  enough  to  render  the 
disc  of  the  sun  red,  always  accompanies  this  wind. 
The  particles  deposited  by  this  fog  upon  the  leaves 
of  vegetables,  and  on  the  black  skin  of  the  natives, 
appears  always  white,  but  the  nature  of  this  whitish 
matter  was  not  ascertained.  It  was  remarked  that 
this  fog  was  speedily  dissipated  by  the  sea;  for, 
although  the  wind  was  sensible  on  the  sea  at  many 
leagues  from  the  coast,  the  fog  became  rapidly  less 
dense,  and,  at  the  distance  of  a  little  more  than  a 
league,  it  disappeared. 

"  One  of  the  characteristics  of  this  wind  and  fog 
is  extreme  dryness.  When  it  continued  for  any 
time,  the  foliage  of  the  orange  and  lemon  trees  ex- 
posed to  it  became  shriveled  and  withered.  So  ex- 
treme is  this  dryness,  that  the  covers  of  books,  even 
when  closed,  locked  in  chests,  and  enveloped  in 
linen  cloth,  were  curved  by  it,  just  as  if  they  had 
been  exposed  to  the  heat  of  a  strong  fire.  The 
panels  of  doors,  and  frames  of  windows,  and  the 
5  D 


50  T  R  E  A  T  I  S  E     0  N     C  0  M  E  T  S  . 

furniture,  were  often  cracked  and  broken  by  it.  Its 
effects  upon  the  human  body  were  not  less  marked. 
The  eyes,  lips,  and  palate  were  parched  and  painful. 
If  the  wind  continued  unabated  so  long  as  four  or 
five  days,  the  face  and  hands  grew  pallid.  The 
natives  endeavored  to  counteract  these  effects  by 
smearing  their  skin  with  grease." 

This  description  may  serve  to  give  a  very  correct 
idea  of  the  character  of  the  Harmattan,  which  we 
conceive  to  be  not  unlike  what  might  be  expected 
in  case  the  earth  should  be  enveloped  in  the  tail  of 
a  comet.  From  what  is  said  of  its  effect  on  the 
human  body,  it  might  be  conjectured  that  its  final 
influence  would  be  highly  insalubrious ;  yet  it  is 
further  related  that  it  proved  to  have  the  extreme 
opposite  quality.  It  was  found  that  its  first  breath 
completely  banished  intermittent  fevers.  It  is  as- 
serted, also,  that  those  who  had  been  enfeebled  by 
the  practice  of  excessive  bleeding  then  prevalent 
there,  soon  recovered  their  strength ;  that  epidemic 
and  remittent  fevers,  which  had  a  local  prevalence, 
disappeared  as  if  by  enchantment;  and  that  it  ren- 
dered infection  incommunicable,  even  when  applied 
by  artificial  means,  such  as  inoculation. 

The  immediate  effect  of  the  passage  of  the  earth 
through  the  tail  of  a  comet,  would  be  to  produce 
unusual  disturbances  of  the  atmosphere,  partly  by 
currents  and  partly  by  electrical  action ;  and  also 


DRY  FOG!  OF  1788.  51 

to  produce  the  most  intense  fogs.  It  has,  therefore, 
been  conjectured  that  the  great  dry  fogs  which 
spread  over  a  large  portion  of  the  surface  of  the 
earth  in  the  years  1783  and  1831,  were  produced  by 
the  near  approach  of  a  comet  to  the  earth,  and  that 
the  tail  of  the  comet  must  have  either  passed  over 
the  earth  or  over  a  part  of  it. 

The  great  dry  fog  of  1783  possessed  several  char- 
acteristics which  have  induced  the  belief  in  its 
cometary  origin ;  and  one  of  these  is  the  fact,  that 
it  commenced  on  nearly  the  same  day  at  places  very 
distant  from  each  other.  It  lasted  nearly  a  month, 
and  it  was  found  that  its  position  was  not  in  the 
least  affected  by  the  winds,  which  proves  that  the 
atmosphere  did  not  convey  it  over  the  regions  in 
which  it  prevailed.  It  was  found,  also,  that  it  pre- 
vailed equally  at  all  accessible  heights  above  the 
surface,  and  that  it  was  as  dense  upon  the  summits 
of  the  Alps  as  upon  the  plains  of  France.  It  caused 
a  general  sensation  throughout  Europe,  and  pre- 
sented the  following  phenomena:  Its  density  was 
such,  that  in  some  places  objects  at  the  distance  of 
a  few  rods  could  not  be  distinguished,  and  even 
such  as  could  be  distinguished  appeared  blue,  or 
else  surrounded  with  vapor.  The  sun  appeared  red, 
and  without  brilliancy,  and  could  be  gazed  at  with 
the  eye  unprotected  at  mid-day ;  while  at  its  rising 
and  setting,  it  disappeared  in  a  sort  of  haze  before 


52  TREATISE   ON   COMETS. 

it  had  approached  within  ten  or  twelve  degrees  of 
the  horizon. 

This  fog  was  first  seen  at  Copenhagen  on  May 
29th,  and  was  preceded  by  a  succession  of  fine  days. 
In  other  places  it  was  preceded  by  a  gale.  In  Eng- 
land it  came  after  continuous  rains  ;  at  La  Kochelle, 
it  was  seen  on  June  6th  and  7th;  at  Dijon,  on  the 
14th.  It  was  noticed  almost  everywhere  in  Ger- 
many, France,  and  Italy,  from  the  16th  to  the  18th ; 
on  the  19th  it  was  observed  at  Franecker  and 
throughout  the  Netherlands ;  on  the  22d,  at  Spyd- 
berg,  in  Norway ;  on  the  23d,  at  St.  Gothard  and  at 
Buda ;  the  24th,  at  Stockholm ;  the  25th,  at  Moscow ; 
toward  the  end  of  June,  in  Syria ;  and,  on  the  1st 
of  July,  in  the  Altai.  It  will  thus  be  perceived  that 
it  "covered  a  part  of  the  earth's  surface,  extending 
north  and  south,  from  Africa  to  Sweden ;  and  we 
might  remark,  in  addition,  that  it  prevailed  on  the 
North  American  as  well  as  upon  the  European 
continent. 

An  examination  of  this  fog  showed  that  it  was 
absolutely  dry,  since  the  most  delicate  hygrometric 
instruments  exposed  in  it  indicated  the  complete 
absence  of  humidity.  It  was  found,  also,  to  have  a 
faintly  luminous  quality,  such  as  might  be  supposed 
to  proceed  from  a  slight  degree  of  phosphorescence  ; 
and  some  observers  maintained  that  they  found 
traces  of  acid  in  it. 


DRY   FOG    OF   1783.  53 

In  order  to  explain  all  these  phenomena,  which 
had  never  before  been  noticed  in  the  case  of  the 
dry  fogs  which  sometimes  prevail  in  different  parts 
of  Germany,  the  cometary  hypothesis,  which  re- 
ferred its  origin  to  the  nebulous  and  slightly  phos- 
phorescent matter  of  the  tail  of  a  comet  which  may 
have  been  at  that  time  passing  over  the  earth,  has 
been  advanced.  To  decide  this  question  definitely 
is  a  problem  which  admits  of  a  ready  and  easy  solu- 
tion, and  one  which  any  one  can  fully  comprehend. 
All  the  phenomena  presented  in  the  case  of  the  fog, 
both  in  respect  to  time  and  place,  we  have  already 
minutely  described,  and  it  remains  only  to  see 
whether  a  comet  could  have  occupied  for  so  long  a 
time  a  position  such  as  the  admission  of  the  come- 
tary hypothesis  would  require.  The  tails  of  comets, 
as  we  shall  see  hereafter,  are  usually  in  a  direction 
opposite  to  the  sun ;  and  it  results,  therefore,  that 
in  order  to  satisfy  the  necessary  conditions,  the 
comet  must  have  been  almost  exactly  in  conjunction 
with  the  sun.  $"ow,  for  reasons  which  it  would  be 
useless  to  attempt  to  explain,  but  which  may  be 
readily  conceived  of,  there  is  no  possible  combina- 
tion of  the  motion  of  the  comet  in  its  orbit,  and  of 
the  diurnal  motion  of  the  earth,  which  would  be 
compatible  with  the  position  and  continuance  of  the 
great  dry  fog  of  1783.  Had  the  interval  during 
which  the  fog  prevailed  been  much  shorter,  and  the 


54  TREATISE   ON    COMETS. 

date  of  its  first  appearance  general,  or  nearly  so, 
the  hypothesis  of  a  cometary  origin  might  be  pre- 
sented with  a  much  greater  show  of  probability; 
but  when  we  remember  that  the  angular  motion  of 
a  comet,  properly  situated,  cannot  be  reconciled 
with  the  continuance  of  any  such  phenomenon  for 
a  period  longer  than  a  very  few  days,  we  can  con- 
clude, in  view  of  all  the  facts,  with  the  utmost  cer- 
tainty, that  the  phenomenon  under  consideration 
did  not  arise  from  the  immersion  of  the  earth  in 
the  tail  of  an  unseen  comet. 

The  great  fog  of  1831  was,  in  every  respect,  ex- 
cepting only  the  extent  of  country  over  which  it 
prevailed,  similar  to  that  of  1783.  It  appeared  in 
the  month  of  August,  commencing  on  the  north 
coast  of  Africa  on  the  3d,  at  Odessa  on  the  9th, 
throughout  France  on  the  10th,  in  the  United  States 
on  the  15th,  and  in  China  during  the  latter  part  of 
the  month.  Observers  in  the  north  of  Africa,  in 
the  south  of  France,  in  the  United  States,  and  in 
China,  reported  that  the  disc  of  the  sun  seen 
through  the  fog  had  the  tint  of  azure,  and,  in  some 
places,  of  emerald  green.  During  the  day  the  light 
blue  of  the  sky  appeared  to  be  tarnished  by  a  mix- 
ture of  a  dirty  color.  At  an  altitude  of  a  few  de- 
grees above  the  horizon,  the  blue  of  the  sky  was 
suddenly  interrupted,  and  was  terminated  above  by 
a  ring  of  dull  brown-red  color,  more  or  less  clearly 


DRY   FOGS.  55 

defined.  Distant  terrestrial  objects  of  a  deep  color 
appeared  to  be  nearly  effaced,  or  covered  by  a  bluish 
veil.  It  was  found,  also,  that  this  fog  had  a  proper 
light,  similar  to  that  described  in  connection  with 
the  great  fog  of  1783,  and  it  was  asserted  that  this 
apparently  phosphorescent  light  was  such  that  in 
the  absence  of  the  moon,  even  at  midnight,  it  was 
possible  to  read  the  smallest  written  or  printed 
characters. 

Another  fog  of  similar  character  appeared  in  the 
months  of  May,  June,  and  July,  1834,  which  was 
confined  almost  exclusively  to  the  western  portion 
of  Germany ;  and  it  is  recorded,  also,  that  similar 
fogs  appeared  in  the  years  526,  1721,  and  1822.  In 
each  of  these  instances,  however,  as  we  have  already 
noticed  in  the  case  of  the  fog  of  1783,  the  hypo- 
thesis of  the  immersion  of  the  earth  in  the  tail  of  a 
comet  is  completely  overthrown  by  the  fact  that  the 
fog,  although  extensively  spread,  was  not  contin- 
uous, and  certainly  not  uniform.  Some  parts  of  the 
European  continent  were  altogether  or  nearly  free 
from  it,  while  in  other  parts  it  was  developed  in  very 
different  degrees.  The  times  of  its  continuance  in 
different  places  also  varied  much  and  irregularly, 
and  in  such  a  manner  as  to  be  entirely  incompatible 
with  the  cometary  hypothesis.  Thus  do  we  find  that 
each  new  case  presented  furnishes  additional  evi- 
dence to  show,  that  however  mysterious  and  unac- 


56  TREATISE   ON    COMETS. 

countable  these  phenomena  may  have  been,  yet  that 
we  are  of  necessity  compelled  to  refer  them  entirely 
to  terrestrial  agency,  and  to  admit  that  they  had  no 
connection  whatever  with  any  celestial  phenomenon. 
At  the  times  when  these  great  fogs  appeared,  the 
supposition  of  cometary  agency  became  very  preva- 
lent ;  but  in  the  inquiry  which  was  excited,  failing 
to  obtain  any  tangible  evidence  in  support  of  such 
a  hypothesis,  various  other  theories  were  devised. 
Lalande  attributed  the  fog  of  1783  to  the  quantity 
of  electricity  developed  during  a  very  hot  summer 
that  succeeded  a  moist  winter ;  while  Cotte  regarded 
it  as  formed  of  metallic  emanations  united  with 
electricity,  in  consequence  of  the  great  heat  and 
numerous  earthquakes  which  marked  the  summer 
of  1783.  It  is  a  historical  fact,  that  in  the  year  in 
which  this  fog  appeared,  there  was  a  very  violent 
earthquake  in  Calabria,  and  a  volcanic  eruption  in 
Iceland,  and  very  many  have  attributed  the  exist- 
ence of  this  fog  to  these.  It  has  been  determined 
by  accurate  researches  that  the  column  which  rises 
above  a  volcano,  although  bearing  a  very  great 
analogy  to  a  column  of  smoke,  is  composed,  for  the 
most  part,  of  the  vapor  of  water  and  of  volcanic 
ashes,  with  which  are  mixed  transparent  gases  in 
various  quantities.  "When  the  lava  runs  over  the 
side  of  the  mountain,  it  carbonizes  everything 
which  it  meets,  and  an  immense  cloud  of  smoke 


DRY   FOGS.  57 

rises  in  the  air.  Considering,  therefore,  the  fact 
that  an  immense  quantity  of  vegetables  were  con- 
sumed in  Iceland,  as  well  as  seventeen  villages,  we 
can  readily  comprehend  that  the  lava,  when  running 
over  a  soil  covered  with  these  vegetables,  might 
have  been  able  to  produce  this  smoke,  which  the 
north  winds,  then  prevalent,  would  immediately 
spread  over  a  great  portion  of  Europe.  Add  to 
this,  that  the  combustions  of  turf  and  the  confla- 
grations of  forests  were  both  extensive  and  frequent 
during  the  summer  of  1783,  which  was  a  very  dry 
one,  and  we  can  conceive  of  the  distribution",  in  a 
very  short  space  of  time,  of  the  vapor,  smoke,  and 
gaseous  matter  combined,  over  nearly  the  entire 
surface  of  the  globe,  and  under  conditions,  as  re- 
spects its  general  characteristics  and  electrical  influ- 
ence, producing  all  the  phenomena  which  were 
observed. 

Such  are  some  of  the  most  prominent  examples 
of  the  supposed  agency  of  comets  in  atmospherical 
disturbances,  by  permeating  the  atmosphere  of  the 
earth  with  their  gaseous  matter ;  and  although,  in 
each  of  these  cases,  this  hypothesis  has  been  found 
to  be  insufficient  for  the  explanation  of  the  phe- 
nomena, yet  we  cannot,  on  this  account,  venture 
the  assertion,  that  absolutely  no  such  event,  as  the 
cometary  hypothesis  under  consideration  would  seem 
to  indicate,  can  ever  take  place.  We  have  already 


58  TREATISE   ON    COMETS. 

seen  that  there  does  exist  a  possibility  of  the  pas- 
sage of  the  tail  of  a  comet  over  the  surface  of  the 
earth,  so  as  to  envelop  either  the  whole  or  a  part  of 
its  surface ;  but  we  have  also  seen  that  the  proba- 
bility of  such  an  occurrence  is  so  small  that  it  can 
hardly  be  expected ;  and  in  proof  of  this  assertion 
we  have  the  fact,  that  during  the  period  of  authentic 
history,  there  is  no  phenomenon  on  record  which 
may  not  be  satisfactorily  explained  without  a  resort 
to  any  other  than  terrestrial  agency,  which  would 
otherwise  be  referred  to  a  cometary  origin. 

Having  thus  disposed  of  cometary  influences,  so 
far  as  they  have  been  supposed  to  relate  to  the  birth 
and  death  of  heroes,  to  the  prevalence  of  famine, 
pestilence,  war,  and  various  other  political  and 
physical  convulsions  which  have  from  time  to  time 
taken  place  in  the  affairs  of  the  world ;  and  having 
shown  conclusively,  as  must  be  admitted,  that  the 
comets  have  no  influence  whatever  over  atmospheric 
changes,  either  directly  or  indirectly,  we  are  pre- 
pared to  consider  the  other  question  which  we  have 
proposed  as  relating  further  to  this  branch  of  our 
subject,  namely,  the  possibility  of  a  collision  be- 
tween a  comet  and  the  earth,  and  the  probable  effect 
of  such  a  collision.  The  question  thus  presented 
is  one  which  admits  of  the  closest  reasoning,  and 
one  in  which  the  results  arrived  at,  admitting  the 
validity  of  the  argument,  must  be  morally  certain. 


POSSIBILITY  OF  A  COLLISION.   59 

We  are  here  not  left  to  the  guidance  of  facts  con- 
tradictory and  ill  authenticated ;  but,  on  the  con- 
trary, the  data  are  both  abundant  and  accurate. 
Under  such  circumstances,  the  case  presented  can- 
not fail  to  be  of  the  greatest  'interest,  inasmuch  as 
it  involves  consequences  which  may  ultimately  prove 
disastrous  to  the  inhabitants  of  the  earth,  if  not  to 
the  existence  of  the  planet  itself  in  its  present  con- 
dition. 

When  we  consider  the  great  number  of  comets 
that  are  known  to  be  continually  traversing  our 
system,  and  also  the  fact  that  the  orbits  of  some  of 
them,  as  we  shall  subsequently  see,  almost  intersect 
the  orbit  of  the  earth,  we  may  not  hesitate  to  admit 
at  once  the  possibility  of  a  collision  between  the 
earth  and  one  of  these  bodies.  The  circumstances 
under  which  such  a  catastrophe  may  happen  —  for 
such  it  would  certainly  be  —  are  these :  The  orbits 
of  the  earth  and  comet  must  either  actually  inter- 
sect each  other,  or  at  least  approach  so  near  at  some 
point,  that  the  distance  between  the  orbits  shall  be 
less  than  the  sum  of  the  radii  of  the  nucleus  or 
head  of  the  comet  and  the  earth ;  and,  the  earth 
and  comet  must  both  arrive  at  this  point  at  nearly 
the  same  instant.  These  conditions  are  indispen- 
sable to  a  collision ;  but  it  would,  indeed,  be  pos- 
sible, without  producing  a  collision,  for  an  approach 
of  a  comet  to  the  earth  to  such  an  extent  that  its 


60  TREATISE   ON    COMETS. 

orbit  would  be  changed,  or  else  it  might  be  drawn 
to  the  earth,  in  ease  the  attractive  force  of  the  latter 
exceeded  that  of  the  sun.  Another  indispensable 
condition  to  the  possibility  of  a  collision,  would  be 
that  the  earth  and  comet  should  move  in  contrary 
directions,  or,  in  other  words,  that  the  motion  of  the 
comet  should  be  retrograde.  The  planets  all  revolve 
around  the  sun  from  west  to  east,  while  the  comets, 
as  we  shall  notice,  are  not  all  found  to  move  in  the 
same  direction.  Some  of  them  move  from  west  to 
east,  in  the  direction  of  the  planetary  motions,  while 
perhaps  an  equal  number  move  from  east  to  west, 
or  in  a  direction  contrary  to  that  of  the  planets. 
Should  the  earth  come  in  contact  with  a  direct 
comet,  they  would  continue  to  move  on  in  new 
orbits,  which  would  necessarily  result  from  their 
impinging ;  and  the  deviation  in  either  case  would 
be  least  when  the  planes  of  the  two  orbits  were 
nearly  coincident,  and  when  they  crossed  each  other 
very  obliquely.  Should  the  orbit,  however,  of  a 
comet  which  moves  with  retrograde  motion,  as 
compared  with  the  planets,  intersect  the  orbit  of  the 
earth,  and  the  comet  and  earth  happened  to  meet  at 
this  point,  the  result  would  be  an  absolute  collision, 
the  effect  of  which  would  certainly  be  disastrous  to 
the  comet,  if  not  to  both  bodies.  Since,  therefore, 
the  conditions  on  which  a  collision  depends  are 
such  as  may  actually  exist,  it  may  not  be  improper 


EFFECT   OF   A   COLLISION.  61 

to  consider  at  once  what  must  be  the  inevitable 
result  of  such  an  event,  before  discussing  the  abso- 
lute probability  of  its  occurrence. 

We  have  just  remarked,  that  in  the  case  of  a 
simple  appulse  between  a  comet  and  the  earth,  or 
in  the  case  that  the  motions  of  the  comet  and  the 
earth  should  both  be  nearly  in  the  same  direction, 
the  effect  produced  would  be  merely  local  in  its 
character,  and  might  perhaps  cause  a  deflection  in 
the  path  of  the  earth,  and  change  its  velocity.  If, 
however,  the  comet  had  no  solid  nucleus  of  any 
considerable  size,  the  effect  either  of  an  appulse,  or 
even  of  a  direct  collision,  would  not  probably  be 
different  from  that  produced  by  the  passage  of  the 
tail  of  a  comet  over  the  earth,  which  we  have  already 
noticed.  -The  effect  produced  in  this  case  would  be 
nothing  more  than  an  intermixture  of  the  cometic 
atmosphere  with  that  of  the  earth.  The  extremely 
light  mass  of  the  comet  would,  notwithstanding  its 
proximity,  render  it  impossible  that  it  could  produce 
any  sensible  effect,  either  on  the  annual  or  diurnal 
motion  of  the  earth ;  and,  consequently,  no  change 
would  be  produced  in  our  days,  seasons,  and  years. 
But  in  the  case  where  the  comet  had  a  solid  nucleus, 
and  withal  moved  in  a  direction  contrary  to  the 
annual  motion  of  the  earth,  the  most  fatal  effects 
would  be  produced  by  a  collision.  The  momentum 
might  indeed  be  sufficient  to  destroy  the  progressive 
6 


62  TREATISE   ON    OOAJKTS. 

motion  of  both  bodies;  and  it  would  result  from 
this  that  the  attractive  force  of  the  sun  would  cause 
both  to  fall  to  his  surface.  Such  would  be  the  fate 
of  the  earth  if  it  were  struck  by  a  cornet,  with  a 
mass  only  about  four  times  that  of  the  moon,  and 
moving  with  a  retrograde  motion  at  the  rate  of 
about  three  hundred  and  fifty  miles  in  a  second  of 
time,  and  both  bodies  would  fall  to  the  sun  in  sixty- 
four  and  a  half  days.  In  case  any  obstruction  had 
destroyed  the  centrifugal  force  of  the  great  comet 
of  1680,  when  nearest  the  sun,  it  would  have  fallen 
to  his  surface  in  three  minutes. 

Let  us,  however,  suppose  a  case  which  aifords  a 
greater  degree  of  probability,  —  since  the  one  just 
supposed  is  such  as  is  almost,  if  not  entirely  impos- 
sible, —  namely,  that  the  shock  experienced  by  the 
earth  from  a  collision  w^as  not  such  as  to  destroy  the 
centrifugal  force.  It  can  readily  be  understood, 
without  further  explanation,  that  the  effect  of  the 
concussion  in  this  case  would  be  to  change  both  the 
axis  around  which  it  performs  its  diurnal  revolution, 
and  the  time  of  rotation.  The  fluids  composing 
the  atmosphere,  the  oceans,  seas,  lakes,  and  rivers, 
not  partaking  of  that  change,  would  be  thrown 
from  their  position  of  relative  equilibrium.  Violent 
atmospheric  commotions  would  ensue,  and  there 
would  also  be  a  sudden  rush  of  all  the  waters  of  the 
ocean,  lakes,  and  rivers  from  their  former  beds, 


EFFECT   OF   A    COLLISION.  63 

overflowing  the  land,  and  sweeping  before  them 
animate  and  inanimate  beings  in  one  vast  and  un- 
distinguishable  ruin.  The  entire  animal,  and  per- 
haps the  vegetable  creation,  would  be  destroyed  by 
a  universal  deluge,  or  by  the  violence  of  the  colli- 
sion. Every  vestige  of  animated  existence  would 
be  forever  obliterated,  and  all  the  monuments  of 
human  industry  overturned.  It  has,  therefore,  been 
asserted  that  in  this  way  we  may  explain  why  the 
ocean  has  deserted  the  most  lofty  mountains,  leav- 
ing, however,  incontestible  evidence  of  its  presence ; 
why  the  animals  and  plants  of  the  tropics  may  have 
existed  in  the  higher  latitudes,  where  their  relics 
and  footsteps  are  still  seen.  It  h^s  further  been 
asserted  that  this  will  explain  the  recent  date  of  the 
present  races  of  men,  whose  earliest  monuments  go 
back  only  a  few  thousand  years ;  since  the  human 
race,  reduced  to  a  small  group  of  individuals,  in  a 
deplorable  condition,  occupied  exclusively  in  pro- 
viding for  their  physical  wants,  must  necessarily 
have  lost  the  remembrances  and  records  of  all  the 
sciences  and  arts ;  and  when,  later,  new  wants  were 
created  by  the  progress  of  civilization,  everything 
was  to  be  recommenced,  as  if  no  previous  progress 
had  been  made,  and  as  if  man  had  been  for  the  first 
time  placed  upon  the  earth. 

It  is  well  known,  that  all  parts  of  the  globe  bear 
testimony  to  the  vast   and  -destructive   floods   of 


64  TREATISE   ON    COMETS. 

mighty  waters.  The  remains  of  plants  and  animals 
deeply  buried  in  the  ground,  and  the  "footprints" 
in  the  solid  rocks,  declare  that  long  before  man 
became  its  inhabitant,  this  earth  of  ours  was  ten- 
anted by  innumerable  races  of  beings  altogether 
different  from  those  which  share  with  us  the  present 
state  of  things.  These  must  have  ceased  to  exist 
as  animated  beings  thousands  and  thousands  of 
years  ago,  since  the  strata  under  which  they  are 
found  indicate  that  sometimes  the  waters  have  pre- 
vailed for  many  centuries  over  the  beds  containing 
them,  and  sometimes'  the  dry  land.  Thus,  too,  do 
we  find  that  enormous  masses  of  rock  have  been 
torn  from  their  native  hills,  and  have  been  carried 
to  distant  regions,  and  these,  together  with  the  de- 
posits of  the  natives  of  the  deep  on  the  tops  of  the 
loftiest  mountains,  declare  the  irresistible  force  and 
magnitude  of  the  vast  waves  which,  in  remote 
times,  have  carried  destruction  over  the  face  of 
nature.  These  and  similar  effects  have  been  attri- 
buted to  the  shock  which  may  have  been  experi- 
enced from  a  comet  in  those  remote  ages  of  the 
world. 

The  appearance  of  the  comet  of  1680  led  "Whiston, 
the  friend  of  Newton,  and  the  man  whom  he  had 
designed  as  his  successor,  to  make  an  attempt  to 
account  for  the  Mosaic  deluge  by  supposing  the 
earth  to  have  been  struck  by  a  comet  at  that  time. 


EFFECT   OF   A   COLLISION.  65 

Not  content  with  simply  conjecturing  the  possibility 
of  explaining  the  deluge  in  this  manner,  he  at- 
tempted further  to  exhibit  a  coincidence  between 
the  theory  and  the  circumstances  of  that  great 
catastrophe,  as  related  in  Genesis. 

The  biblical  deluge  happened  in  the  year  2349 
before  the  commencement  of  our  era,  according  to 
the  modern  Hebrew  text ;  and  in  the  year  2926,  ac- 
cording to  the  Septuagint  and  Josephus.  Whiston, 
therefore,  endeavored  to  prove  that  a  great  comet 
may  be  supposed  to  have  appeared  at  that  time. 

It  is  related,  that  a  large  comet  appeared  in  the 
year  1106,  which  resembled  the  blaze  of  the  sun, 
and  had  an  immense  train.  In  ascending  still 
higher,  we  find  a  very  large  and  terrific  comet, 
designated  by  the  Byzantine  historians  by  the  name 
of  Lampadius,  whose  appearance  must  have  been 
in  the  year  531  of  our  era.  A  comet  also  appeared 
in  the  month  of  September,  of  the  year  43  before 
our  era,  shortly  after  the  death  of  Julius  Csesar, 
which  is  said  to  have  been  very  brilliant,  and  to 
have  been  visible  for  some  time  before  sunset. 

A  comparison  of  these  dates  reveals  the  fact  that, 
supposing  these  to  have  been  successive  appearances 
of  the  same  comet, — which,  it  should  be  remarked, 
is  altogether  conjectural, —  the  period  of  revolution 
of  the  comet  of  1680  differs  but  little  from  575 
years.  Adopting  this  hypothesis,  and  computing 


66  TREATISE   ON    COMETS. 

backward,  we  find  that  four  revolutions  prior  to 
B.  C.  43,  would  give  the  epoch  of  the  deluge,  ac- 
cording to  the  modern  Hebrew  text,  within  six 
years ;  and  by  supposing  that  there  had  been  five 
revolutions,  we  obtain  the  date  of  the  deluge,  ac- 
cording to  the  Septuagint,  within  eight  years.  From 
these  and  other  considerations,  it  has  been  inferred 
that,  as  Whiston  has  supposed,  there  may  have 
been  a  comet  near  the  earth  at  the  period  of  the 
deluge,  although  the  probability  of  its  having  been 
in  collision  admits  of  no  such  assumption.  It  ought 
also  to  be  remarked  that  the  comet  of  1680  moved 
in  the  same  direction,  relatively,  in  which  the  earth 
moves ;  yet  the  plane  of  its  orbit  was  so  inclined 
to  that  of  the  earth's  orbit, that  a  collision,  although 
barely  possible,  yet  could  not  have  been  so  disas- 
trous as  might  have  resulted  in  case  the  motion  had 
been  retrograde. 

But  without  entering  into  a  detailed  account  of 
the  arguments  adduced  by  "Whiston  in  support  of 
his  hypothesis,  it  will  perhaps  be  sufficient  to  notice 
only  the  general  conclusions  which  he  arrived  at. 

He  asserts,  that  at  the  period  of  the  deluge,  the 
comet  of  1680  was  only  nine  or  ten  thousand  miles 
from  the  earth ;  and,  consequently,  that  it  attracted 
the  waters  from  the  great  deep,  just  as  the  moon  is 
known  to  attract  the  waters  of  the  ocean.  Its  action, 
on  account  of  that  great  proximity,  must  have 


EFFECT   OF   A   COLLISION.  67 

tended  to  produce  an  immense  tide ;  and  hence,  he 
contends,  that  since  it  is  probable  that  the  terrestrial 
shell  could  not  resist  the  impetuosity  of  the  inun- 
dation, it  would  break  in  at  a  great  number  of 
points,  and  the  waters,  then  free,  would  spread 
themselves  over  the  continents.  To  render  this 
intelligible,  it  might  be  well  to  state  that  he  sup- 
posed the  earth  to  have  been  originally  a  comet. 
He  therefore  regarded  it  as  having,  at  or  near 
its  centre,  a  nucleus,  which  is  a  hard  and  compact 
substance,  and  which  was  formerly  the  nucleus  or 
head  of  the  comet.  He  supposed,  also,  that  the 
matters  of  various  natures,  confusedly  mixed,  which 
composed  the  nebulosity,  subsided  more  or  less 
quickly,  according  to  their  specific  gravities ;  that 
the  then  solid  nucleus  was  at  first  surrounded  by  a 
dense  and  thick  fluid;  that  the  earthy  matters  pre- 
cipitated themselves  afterwards  and  formed  a  cover- 
ing over  the  dense  fluid  —  a  kind  of  crust,  which 
may  be  compared  to  the  shell  of  an  egg ;  that  the 
water,  in  its  turn,  covered  this  solid  crust ;  that  in 
a  considerable  degree  it  became  filtered  through  the 
fissures,  and  spread  itself  over  the  thick  fluid ;  and 
that,  finally,  the  gaseous  matters  remaining  sus- 
pended, purified  themselves  gradually,  and  consti- 
tuted our  atmosphere.  Thus  he  makes  it  possible, 
as  he  imagines,  for  the  supposed  shell  to  be  broken 
up  by  the  rush  of  the  exterior  waters;  and  when 


68  TREATISE   ON    COMETS. 

once  broken  up,  the  waters  which  he  supposed  to 
have  existed  interior  to  this  crust,  and  on  which  he 
supposed  it  to  rest,  would  gush  forth,  and  produce 
the  phenomena  given  in  Genesis,  namely:  "In  the 
second  month,  the  seventeenth  day  of  the  "month, 
the  same  day  were  all  the  fountains  of  the  great 
deep  broken  up." 

Continuing  his  theory  still  further,  Whiston  sup- 
posed that  the  nebulosity  of  the  comet  reached  the 
earth  near  Mount  Ararat,  arid  that  the  range  of 
mountains  intercepted  the  entire  tail.  He  con- 
cludes, therefore,  that  it  would  result,  as  a  necessary 
consequence,  that  the  terrestrial  atmosphere,  thus 
charged  with  an  immense  quantity  of  aqueous  par- 
ticles, was  sufficient  to  produce  forty  days'  rain  of 
such  violence  as  the  ordinary  state  of  the  globe  can 
give  us  no  idea.  Thus  he  reconciles  the  remainder 
of  the  passage  found  in  Genesis:  "And  the  win- 
dows of  heaven  were  opened.  And  the  rain  was 
upon  the  earth  forty  days  and  forty  nights." 

Such  is  the  celebrated  theory  of  "Whiston  in  rela- 
tion to  the  manner  in  which  the  Mosaic  deluge  was 
accomplished ;  a  theory  which  is  remarkable  from 
the  fact  that  it  was  advanced  and  maintained  by  a 
man  who  was  an  intimate  friend  of  the  great  New- 
ton, and  who  must  have  been  acquainted  with  the 
labors  of  that  distinguished  philosopher.  In  many 
points  this  theory  may  seem  to  present,  at  least,  some 


EFFECT   OF   A   C  o  L  L  i  n  ION.  69 

show  of  probability,  but  there  are  so  many  other 
points  in  which  it  presents  obvious  and  glaring 
defects,  that  we  can  regard  it  as  nothing  more  than 
the  fancy  of  a  mind  devoted  exclusively  to  similat 
speculations.  Geologists  have  shown  that  the  earth 
has  indeed  an  exterior  crust  or  shell,  the  thickness' 
of  which  is  very  small  compared  with  the  diameter 
of  the  globe,  but  that  the  interior,  instead  of  being 
in  the  state  supposed  by  Whiston,  is,  on  the  con- 
trary, in  a  state  of  intense  heat,  such,  in  fact,  that 
the  great  bulk  is  one  vast  mass  of  melted  matter. 
This  militates  strongly  against  the  theory  of  Whis- 
ton ;  and  were  there  not  additional  considerations 
to  counterbalance,  would  be  sufficient  to  show  the 
utter  fallacy  of  all  his  arguments  in  relation  to  the 
deluge. 

In  order  to  give  a  greater  weight  to  his  argu- 
ments, and  at  the  same  time  to  remove  objections 
which  would  otherwise  serve  to  lessen  materially 
the  strength  of  his  theory,  Whiston  supposed  that 
the  mass  of  the  comet  was  at  least  six  times  greater 
than  that  of  the  moon.  It  can,  however,  be  readily 
shown  that  even  this  enormous  magnitude  of  the 
comet  would  not  be  sufficient  to  produce  any  such 
tides  as  he  supposes  to  have  been  caused ;  and  that 
the  comet  could  not,  by  any  possibility,  have  re- 
mained more  than  two  hours  and  a  half  so  near  the 
earth  as  a  fourth  part  of  the  moon's  distance.  It 


70  TREATISE   ON   COMETS. 

may  also  be  shown  that  it  could  not  have  remained 
even  for  that  length  of  time,  unless  it  passed  the 
earth  under  a  very  improbable  and  peculiar  combi- 
nation of  circumstances.  Under  such  circumstances, 
the  production  of  a  tide  would  be  impossible,  since 
eleven  hours  at  least  would  be  necessary  to  enable 
a  comet  to  produce  an  effect  on  the  waters  of  the 
earth,  from  which  the  injurious  consequences  of  a 
deluge  could  follow. 

Thus  we  see  how  utterly  untenable  are  the  doc- 
trines of  Whiston  in  relation  to  the  deluge.  They 
are,  perhaps,  not  less  romantic  than  another  theory 
of  the  same  man,  of  the  probable  object  of  the 
comets  in  the  economy  of  nature.  He  fixed  on 
these  erratic  worlds  the  residence  of  the  damned, 
and  thus  determined  the  nature  of  their  punish- 
ment. According  to  his  theory,  a  comet  was  the 
awful  prison-house  in  which,  as  it  wheeled  from  the 
remotest  regions  of  darkness  and  cold  into  the  very 
vicinity  of  the  sun,  hurrying  its  wretched  tenants 
to  the  extremes  of  perishing  cold  and  devouring 
fire,  the  Almighty  was  to  dispense  the  severities  of 
his  justice. 

Remarkable,  then,  as  the  theory  of  Whiston  is 
found  to  be,  it  is  a  fact  well  established,  that  it  was 
received  with  a  great  deal  of  favor  at  the  time  when 
it  was  promulgated,  and  many  others  following  in 


EFFECT   OF   A    COLLISION.  71 

his  footsteps  have  modified  it,  and  extended  it  to 
suit  their  own  peculiar  fancies. 

If,  however,  we  suppose  the  earth  to  have  received 
a  shock  from  a  collision  with  a  comet  of  any  con- 
siderable magnitude,  it  would  not  he  difficult  to 
conceive  of  a  deluge  such  as  described  in  Genesis. 
Some  writers  have  adopted  this  hypothesis,  and 
have  carried  it  still  further,  and  suppose  the  floods 
of  mighty  waters  w^hich  have  left  their  mark  every- 
where on  the  surface  of  the  glohe,  have  been  pro- 
duced by  the  shock  of  a  comet  in  former  ages.  But 
the  astronomer  has  shown  that  such  is  not  the  case  ; 
that  the  length  of  the  day,  which  is  the  measure  of 
the  celestial  motions,  is  immutable,  and  exhibits  no 
trace  of  change ;  and  that,  if  the  earth  had  ever 
been  struck'  by  a  comet  so  as  to  change  the  axis 
about  which  it  performs  its  diurnal  rotation,  the 
effects  would  still  be  perceptible  in  the  variation 
which  it  would  have  occasioned  in  the  geographical 
latitudes.  It  may  be  demonstrated,  however,  that 
a  spheroid,  such  as  the  earth  is  known  to  be,  cannot 
permanently  revolve  around  any  axis  except  its 
shortest  diameter,  in  case  it  was  placed  in  any  re- 
sisting medium,  such  as  our  atmosphere.  Now,  if 
by  the  collision  of  a  solid  comet,  the  earth  were 
made  to  revolve  on  any  other  diameter  than  its 
shortest,  —  which  is  the  one  around  which  observa- 
tion proves  that  it  now  revolves,  —  it  would  seem, 


72  TREATISE   ON   COMETS. 

for  a  reason  similar  to  that  just  noticed,  that  it  could 
not  continue  so  to  revolve,  and  that  it  ought  to 
change  its  axis  from  hour  to  hour,  until  at  length  it 
would  again  revolve  round  its  shortest  diameter. 
It  is,  however,  certain  that  the  axis  of  the  earth's 
rotation  has  never  been  changed,  because,  as  the 
ethereal  fluid  which  is  supposed  to  pervade  space 
offers  no  sensible  resistance  to  so  dense  a  body  as 
the  earth,  the  libration  would  to  this  day  be  evident 
in  the  variation  of  geographical  latitudes.  As 
nothing  is  known  of  the  primitive  velocity  of  the 
earth,  it  is  possible  that  a  comet  may  have  given  it 
a  shock  without  changing  the  axis  of  rotation,  but 
only  destroying  a  part  of  its  tangential  velocity,  so 
as  to  diminish  the  size  of  the  orbit.  In  this  case 
the  effect  would  have  been  to  increase  the  temper- 
ature of  the  seasons ;  but  geologists  have  shown, 
that  instead  of  the  earth  having  become  warmer, 
the  tropical  nature  of  the  fossil  remains  found  in 
the  most  northern  countries  of  Europe  and  America, 
declare  that  the  general  climate  of  the  earth  is  of  a 
lower  temperature  now,  than  it  was  in  the  extremely 
remote  ages  in  which  these  plants  and  animals  must 
have  flourished. 

We  see,  therefore,  that  the  consequences  of  a  col- 
lision between  a  comet  and  the  earth  might  be 
frightful  in  the  extreme ;  yet  it  may  be  demonstrated 
that  there  is  but  little  chance  that  such  a  collision 


THEORY  OF   COMETARY   INFLUENCE.  73 

can  ever  take  place.     Indeed,  it  has  been  found  by 
actual  calculation,  from  the  theory  of  probabilities, 
that  if  the  nucleus  of  a  comet  having  a  diameter 
equal  only  to  one  fourth  part  of  that  of  the  earth, 
should  come  nearer  the  sun  than  the  earth,  its  orbit 
being  otherwise  unknown,  the  probability  of  the 
earth  receiving  a  shock  from  it,  is  only  one  in  two 
hundred  and  eighty-one  millions.     It  is  found,  also, 
that  the  chance  of  our  coming  in  contact  with  its 
nebulosity  is  not  more  than  ten  or  twelve  times 
greater.     We   may   rest   satisfied,   therefore,   that, 
although  it  cannot  be  affirmed  absolutely  that  the 
earth  will  never  come  in  collision  with  a  comet,  there 
is  no  reasonable  cause  to  dread  such  an  event,  since, 
by  supposing  the  number  of  comets  which  pass  near 
enough  to  the  sun  to  satisfy  the  required  conditions 
to  be  three  in  each  year,  which  is  perhaps  too  large 
an  estimate,  we  arrive  at  the  conclusion  that  the 
earth  may  receive  a  shock  from  one  of  these  bodies 
in  the  course  of  the  next  ninety-four  millions  of 
years,  while  the  probability  is  nearly  as  great  that 
such  an  event  will  never  occur.    This  result,  although 
to  those  unacquainted  with  the  mathematical  theory 
of  probabilities  it  may  seem  conjectural,  is  worthy 
of  the  very  highest  confidence  ;  and  we  may  unhesi- 
tatingly declare  that  although,  in  strict  geometrical 
rigor,  it  is  not  physically  impossible  that  a  comet 
7 


74  TREATISE   ON    COMETS. 

should  encounter  the  earth,  yet  the  moral  possibility 
of  such  an  event  is  absolutely  nothing. 

The  only  instance  in  which  any  such  serious 
catastrophe  has  been  anticipated,  to  any  considerable 
extent,  was  in  the  case  of  the  return  of  the  comet 
of  Biela  to  its  perihelion,  or  point  nearest  the  sun, 
in  the  year  1832.  A  detailed  account  of  this  comet 
will  be  given  in  the  proper  connection ;  but  it  may 
be  well  to  state  here,  that  this  appearance  of  the 
comet  excited  the  liveliest  interest  throughout  the 
civilized  world.  A  short  time  before  its  appearance, 
Damoiseau  created  very  serious  apprehension  in 
France  and  elsewhere,  by  predicting  that  the  comet 
would  pass  within  eighteen  thousand  four  hundred 
and  eighty-four  miles  of  the  earth's  orbit,  a  little 
before  midnight,  on  the  29th  of  October,  1832 ;  and 
since  Olbers  had  computed  that  the  radius  of  the 
comet's  nucleus  or  head  —  which  is  the  distance 
from  the  centre  of  the  comet  to  its  surface — would 
be  twenty-one  thousand  one  hundred  and  twenty 
miles,  it  was  evident  that  its  nebulosity  would  en- 
velop a  portion  of  the  earth's  orbit,  and  if,  by  an}- 
cause,  the  comet  should  be  retarded  one  month  in 
its  revolution,  it  would  undoubtedly  meet  the  earth 
at  that  point.  This  startling  announcement,  ema- 
nating from  a  source  which  could  not  be  questioned, 
excited  the  popular  apprehension  to  the  very  highest 
degree. 


THEORY  OF  C  o  M  E  T  A  R  Y  INFLUENCE.    75 

Arago,  however,  dispelled  the  fears  of  his  coun- 
trymen, by  showing  that  the  comet  would  not  be 
retarded  in  its  course,  but  that  it  would  pass  nearest 
to  the  earth,  at  a  distance  of  fifty-five  millions  of 
miles ;  and,  consequently,  could  not  have  the 
slightest  influence  in  terrestrial  affairs,  since  it  had, 
in  1805,  been  ten  times  nearer  without  any  sensible 
effect,  and  without  exciting  any  alarm. 

We  have  now  completed  the  account  of  cometary 
influences,  both  real  and  imaginary,  and^more  espe- 
cially the  latter ;  and  what  has  been  said  ought  to 
be  sufficient  to  assure  every  intelligent  person,  that 
every  superstitious  notion  which  may  have  been 
attached  to  them,  every  baneful  influence  which 
they  may  have  been  supposed  to  exert,  every  sup- 
posed influence  in  producing  atmospheric  disturb- 
ances, and  in  changing  or  modifying  the  temperature 
of  our  climate,  and  further,  that  all  fears  of  a  colli- 
sion between  one  of  these  bodies  and  our  earth, 
must  be  regarded  as  one  of  the  aberrations  of  the 
human  mind.  We  have  been  able  to  trace,  how- 
ever imperfectly  it  may  have  been,  some  distant 
resemblance  at  least  between  these  notions  of  come- 
tary influence  and  the  development  of  the  philo- 
sophy of  a  people ;  and  we  have  noticed  that  there 
is  even  a  tendency  in  the  human  mind  to  cling  to 
all  such  ideas,  no  matter  how  absurd,  after  *the 
inauguration  of  a  true  philosophy  has  revealed  their 


76  TREATISE   ON   COMETS. 

glaring  inconsistencies.  At  this  very  day  we  read 
in  the  newspapers  often  of  some  comet  which  is 
certainly  about  to  destroy  the  present  state  of 
things ;  while,  even  in  case  no  such  apprehension  is 
announced,  we  are  charmed  with  the  cheerful  intel- 
ligence that  in  the  German  vineyards,  in  the  beau- 
tiful valleys  of  the  Rhine  and  Moselle,  these  once 
ill-omened  bodies  have  been  exercising  a  beneficial 
influence  on  the  ripening  of  the  vine. 

It  is  indeed  pleasing  and  instructive  to  consider 
the  important  facts  which  history  has  furnished  us 
on  this  subject;  but  the  time  has  now  come  when 
all  such  fears  as  we  have  noticed  should  be  at  an 
end,  and  when  the  return  of  a  comet,  which  formerly 
spread  dismay  and  terror  throughout  the  world, 
should  be  regarded  as  one  of  the  greatest  triumphs 
of  science. 

"  Lo !  from  the  dread  immensity  of  space 
Returning  with  accelerated  course, 
The  rushing  comet  to  the  sun  descends; 
And  as  he  sinks  below  the  shading  earth, 
"With  awful  train  projected  o'er  the  heavens, 
The  guilty  nations  tremble.     The  enlighten'd  few 
"Whose  godlike  minds  philosophy  exalts, 
The  glorious  stranger  hail.     They  feel  a  joy 
Divinely  great ;  they  in  their  powers  exult, 
That  wondrous  force  of  thought,  which  mounting  spurns 
This  dusky  spot,  and  measures  all  the  sky ; 
While  from  his  far  excursion  through  the  wilds 
Of  barren  ether,  faithful  to  his  time, 
They  see  the  blazing  wonder  rise  anew." 


APPARENT  FORMS   OF   COMETS.        77 


CHAPTER  II. 

APPARENT  FORMS  OF  COMETS  —  DESCRIPTION  OF  SOME  OF  THE  HOST 
REMARKABLE  COMETS  WHICH  HAVE  APPEARED  DURING  THE  PERIOD 
OF  AUTHENTIC  HISTORY  —  THE  GREAT  COMETS  OF  1556,  1680,  1769, 
1807,  1811,  1843,  1853,  AND  1858  —  NUMBER  OF  COMETS. 

BEFORE  proceeding  to  give  a  detailed  account  of 
some  of  the  most  remarkable  comets  which  have 
appeared  during  the  period  which  has  elapsed  since 
the  earliest  records  of  the  Chinese,  it  may  not  be 
improper  to  explain,  in  this  connection,  some  of  the 
general  and  more  prominent  characteristics  by  which 
these  bodies  are  distinguished.  We  shall  subse- 
quently enter  into  a  complete  exposition  of  the 
theory  of  their  form  and  physical  constitution,  and 
we  propose  to  notice  now  simply  what  may  be 
required  to  render  the  descriptions  which  follow 
intelligible. 

The  comets,  for  the  most  part,  present  the  appear- 
ance of  a  globular  mass  of  illuminated  vapor  or 
nebulous  matter,  with  a  train,  or  brush,  as  it  is 
termed  by  the  Chinese  astronomers,  extending  in  a 
direction  opposite  to  the  sun.  There  are,  however, 
those  which,  being  invisible  to  the  naked  eye,  ex- 
hibit no  signs  of  a  train  or  elongation  opposite  the 


78  TREATISE   ON   COMETS. 

sun,  and  of  which  even  the  nebulous  mass  itself  is 
ill-defined  and  irregular.  The  latter  are  generally 
of  extreme  faintness,  and  evince  their  cometary 
nature  only  by  their  motion  in  their  orbits.  The 
discovery  of  these  has  been  much  more  rapid  of 
late  years  than  formerly,  owing  to  the  perfection 
and  number  of  the  instruments  employed  in  search- 
ing for  them;  and  although,  as  will  hereafter  be 
noticed,  these  apparently  insignificant  objects  are  by 
no  means  uninteresting,  yet  it  will  be  sufficient  for 
our  present  purpose  to  understand  the  phenomena 
presented  in  the  case  of  the  larger  and  brighter 
telescopic  comets,  and  those  visible  to  the  naked 
eye. 

The  nebulous  mass  of  matter,  independent  of  the 
train,  is  called  the  head  of  the  comet,  while  the 
latter  is  usually  termed  the  tail.  At  or  near  the 
centre  of  the  head  there  is  almost  always  a  point 
of  greater  condensation  of  light  than  in  the  other 
parts  of  the  comet ;  and  this  bright  spot,  which  is 
not  invariable,  is  called  the  nucleus.  The  nucleus 
appears,  in  the  case  of  some  of  the  great  comets, 
sometimes  like  a  minute  stellar  point,  and  some- 
times presents  the  appearance  of  a  planetary  disc 
shining  through  a  nebulous  haze,  which  is  termed 
the  coma. 

These  definitions  will  serve  to  illustrate  the  de- 
scriptions which  we  shall  now  give  of  some  of  the 


DESCRIPTION   OF   COMETS.  79 

most  remarkable  comets  whose  appearances  have 
been  recorded ;  and,  as  we  have  already  remarked, 
we  shall  subsequently  explain,  in  the  proper  con- 
nection, all  the  various  changes  which  take  place  in 
the  figure  and  constitution  of  the  nucleus  and  train 
as  the  comet  approaches  the  sun,  and  in  receding 
from  it. 

The  Chinese  have  recorded  the  appearances  of 
comets  professedly  as  far  back  as  the  year  2550 
before  the  commencement  of  our  era;  but  the 
accounts  given  are  so  vague  and  unsatisfactory,  and 
the  dates  so  uncertain,  that  although  these  bodies 
have  not  failed  to  attract  a  great  deal  of  attention 
even  in  the  very  remotest  ages  of  the  world,  yet  we 
have  no  authentic  observations  earlier  than  about 
four  hundred  years  before  Christ.  There  can  be  no 
doubt,  however,  but  that  the  Chinese  have  actual 
records  far  anterior  to  this  date ;  but  so  little  is 
known  in  regard  to  them,  that  we  can  glean  nothing 
from  the  accounts  given  which  could  interest  a 
general  reader. 

During  the  four  centuries  which  immediately  pre- 
ceded the  birth  of  Christ,  we  have  reliable  accounts 
of  the  appearances  of  thirteen  comets.  Some  of 
these  are  described  as  having  been  of  prodigious 
magnitudes,  and  as  having  presented  terrific  aspects. 
So  little  was  known  in  those  days  in  regard  to  their 
nature  and  motions,  that  the  phenomena  recorded 


80  TREATISE   ON   COMETS. 

give  us  no  additional  information  in  regard  to  them 
which  is  of  any  value.  As  usual,  they  were  sup- 
posed to  have  caused  the  most  dreadful  disasters 
which  occurred  at  or  near  the  date  of  their  appear- 
ance ;  and  if  the  accounts  are  not  exaggerated,  the 
phenomena  presented  were  fully  as  imposing  as  any 
which  have  been  exhibited  in  modern  times.  It  is 
related  by  Diodorus,  that  at  the  time  of  the  expe- 
dition of  Timoleus  of  Corinth  to  Sicily,  in  the  year 
344  before  our  era,  a  burning  torch  was  visible  in 
the  heavens  throughout  the  entire  night,  and  that 
it  preceded  the  train  of  Timoleus  until  its  arrival 
in  Sicily,  thus  connecting  indirectly  the  appearance 
of  the  comet  with  this  particular  event.  Seneca 
records  the  appearance  of  a  comet  in  the  year  156, 
which  is  said  to  have  been  apparently  as  large  as 
the  sun.  At  first  its  appearance  was  red  and  fiery, 
emitting  a  suificient  quantity  of  light  to  dissipate 
the  darkness  of  night ;  but  it  afterwards  diminished 
gradually,  and  finally  became  invisible. 

The  year  136  B.  C.  was  signalized  by  the  appear- 
ance of  three  large  comets,  one  of  which  was  so 
bright  that  the  heavens  appeared  to  be  on  fire,  the 
comet  being  described  as  having  occupied  one-fourth 
part  of  the  sky.  Lucian  informs  us  that  a  comet, 
which  appeared  in  the  year  48  B.  C.,  was  so  brilliant 
that  the  darkness  of  the  night  was  illuminated  by  it, 
and  that  the  comet  itself  presented  a  terrific  appear- 


DESCRIPTION   OF   COMETS.  81 

ance.  It  is  also  recorded  that  a  comet  appeared  in 
the  year  43  B.  C.,  which  was  visible  two  or  three 
hours  before  sunset.  Dion  Cassius  relates  that, 
under  the  consulate  of  M.  Valerius  Messala  Barba- 
tus  and  P.  Sulpicius  Quirinus,  —  which  corresponds 
to  the  year  11  B.C., —  before  the  death  of  Agrippa, 
a  comet  was  seen  for  many  days.  It  appeared  as  if 
suspended  over  the  city  of  Rome,  and  afterwards 
resolved  itself  into  many  smaller  comets.  The  latter 
statement  might  cause  us  to  feel  inclined  to  reject 
the  account  given  by  Dion  Cassius,  were  it  not  that 
we  have  already  an  example  of  a  similar  character 
in  the  case  of  Biela's  comet,  which  will  be  described 
hereafter. 

The  foregoing  descriptions  relate  to  the  principal 
comets  which  appeared  anterior  to  the  commence- 
ment of  our  era.  During  the  five  centuries  imme- 
diately following  this  epoch,  we  have  records  of  the 
appearances  of  no  less  than  eighty-three  comets, 
some  of  which  are  described  as  having  been  truly 
magnificent.  A  very  beautiful  one  appeared  in 
China,  in  the  year  178.  It  was  first  seen  on  the  30th 
of  August,  and  exhibited  a  tail  only  a  few  degrees 
in  length.  It  became,  however,  gradually  brighter, 
until  the  train  extended  over  more  than  sixty  de- 
grees of  the  heavens.  The  nucleus  was  remarkable 
for  its  ruddy  appearance,  and  remained  visible  nearly 
three  months ;  having,  in  the  meantime,  passed  over 

F 


82  TREATISE   ON   COMETS. 

an  arc  of  three  hundred  degrees  in  its  apparent 
course. 

In  the  year  389,  a  comet  of  great  brilliancy  ap- 
peared in  the  north-east,  a  few  hours  before  sunrise. 
It  is  described  as  resembling  a  lamp,  with  the  flame 
tending  upwards  towards  the  zenith.  It  was  visible 
during  a  period  of  forty  days,  and  finally  disappeared 
in  the  constellation  Ursa  Major.  A  comet  of  pre- 
cisely the  same  aspect,  and  moving  in  the  same  di- 
rection, was  visible  in  the  following  year.  Claudian 
describes  a  comet  which  was  visible  in  the  year  402, 
as  being  first  seen  in  the  east,  towards  that  part  of 
the  heavens  in  which  the  constellations  Cepheus 
and  Cassiopeia  are  situated.  It  subsequently  moved 
northward  above  Ursa  Major,  and  was  so  brilliant 
that  it  diminished  the  beauty  of  the  bright  stars  of 
that  constellation.  It  then  began  to  fade  away,  and 
finally  vanished  in  a  very  narrow  flame. 

On  the  19th  of  July,  418,  there  was  a  total  eclipse 
of  the  sun,  during  which  the  darkness  was  so  great 
that  the  stars  were  plainly  visible.  During  the  time 
of  total  obscuration,  a  light  in  the  form  of  a  cone 
was  perceived  in  the  sky,  which  resembled  the  flame 
of  a  lamp.  This  object  proved  to  be  a  comet  which 
was  subsequently  seen  in  China,  where  it  presented 
a  very  beautiful  appearance.  When  it  first  became 
visible  in  China,  it  was  in  the  constellation  Cygnus, 
and  moving  west  and  north  through  the  Great  Bear: 


DESCRIPTION   OF   COMETS.  83 

it  finally  disappeared  in  the  constellation  Leo,  having 
been  visible  more  than  three  months. 

After  the  commencement  of  the  sixth  century, 
the  recorded  appearances  of  comets  are  much  more 
frequent;  and  we  shall  attempt  to  notice  only  those 
which  were  most  remarkable,  either  on  account  of 
their  extreme  brilliancy,  or  from  some"  peculiar  phy- 
sical characteristic.  It  should  be  remarked,  how- 
ever, that  until  about  the  year  1200,  we  are  com- 
pelled to  rely  almost  entirely  on  the  Chinese  ac- 
counts, which  are  often,  if  not  generally,  vague  and 
unsatisfactory.  The  great  social  and  political  con- 
vulsions which  were  continually  disturbing  the  na- 
tions of  Europe,  were  hostile  to  the  advancement 
of  astronomical  scien-ce ;  and  while  the  most  en- 
lightened nations  of  the  globe  were  absorbed  wholly 
in  the  study  of  eloquence,  poetry,  and  the  fine  arts, 
and  more  especially  in  the  pursuit  of  military  fame, 
it  was  reserved  for  the  inhabitants  of  the  East  to 
cherish  science  in  its  infancy,  and  to  furnish  the  ele- 
ments of  its  progress.  When  the  affairs  of  Europe 
had  become  settled,  the  study  of  astronomy  soon 
began  to  engage  the  attention  of  learned  men ;  and 
henceforward  we  are  provided  with  the  most  com- 
plete and  accurate  descriptions  of  the  comets  which 
appeared. 

In  the  year  530,  a  very  large  and  brilliant  comet 
appeared  in  China,  which  was  found  to  move  rapidly 


84  TREATISE   ON   COMETS. 

toward  the  north,  and  west.  It  was  first  seen  in 
October ;  and,  since  a  comet  was  seen  in  Europe  in 
the  beginning  of  the  year  531,  presenting  the  same 
general  appearance,  we  may  conclude  that  the  comet 
was  visible  in  China  before  its  passage  through  the 
point  of  its  orbit  nearest  the  sun,  and  in  Europe 
after  its  passage  through  that  point.  This  comet 
has  been  supposed  to  be  identical  with  that  which 
appeared  at  the  death  of  Julius  Csesar,  and  which 
Whiston  supposed  to  have  caused  the  Mosaic  deluge, 
as  we  have  already  noticed.  It  is  related  by  Gregory 
of  Tours,  that  in  the  year  582,  on  Easter  day,  at 
Soissons,  the  heavens  were  seen  on  fire,  —  which 
remark  is  supposed,  in  all  probability,  to  refer  to  a 
very  large  and  terrific  comet,  which  first  made  its 
appearance  in  January  of  that  year.  It  was  sur- 
rounded by  an  immense,  nebulous  envelope,  such 
that  the  comet  is  said  to  have  been  situated,  as  it 
were,  in  a  kind  of  opening.  Its  train  extended  to 
an  enormous  distance  from  the  nucleus,  and  is  de- 
scribed as  having  resembled  the  smoke  of  a  great 
conflagration,  viewed  from  a  distance.  The  comet 
was  seen  in  the  west  immediately  after  sunset. 

On  the  8th  of  July,  615,  a  comet  was  seen  in  China, 
in  the  Great  Bear.  Its  tail  was  fifty  or  sixty  degrees 
in  length,  and  was  remarkable  for  its  dusky  color. 
During  the  night  its  head  or  nucleus  appeared  to 
have  a  motion  of  libration.  It  moved  to  the  north- 


DESCRIPTIONOF   COMETS.  85 

west  for  several  days,  and  afterwards  retrograded 
and  disappeared.  A  comet  is  mentioned  by  Euro- 
pean historians  about  the  time  of  the  election  of 
Pope  Donus,  in  the  year  676,  which  had  a  tail  five 
degrees  in  length,  and  was  remarkable  from  the  fact 
that  its  motion  was  such  that  its  gradual  diminution 
from  day  to  day,  as  the  comet  receded  from  the 
earth,  was  distinctly  noticed.  This  comet  was  also 
observed  in  China,  on  the  4th  of  September,  676,  in 
the  constellation  G-emini.  It  moved  toward  the 
north-east,  and  disappeared  about  the  1st  of  No- 
vember in  the  constellation  Ursa  Major. 

Father  de  Maille  relates,  that  on  the  22d  of  March, 
837,  a  comet  was  seen  in  China  in  the  adjacent  part 
of  the  constellations  Aquarius  and  Pegasus,  the  tail 
of  which  was  seven  degrees  in  length.  It  was  found 
to  move  toward  the  west,  and  on  the  6th  of  April 
the  length  of  the  tail  was  ten  degrees.  On  the  10th 
of  April  the  length  of  the  tail  had  increased  to  fifty 
degrees,  and  was  separated  into  two  distinct  parts, 
the  bifurcation  commencing  at  the  nucleus  and  in- 
creasing in  width  to  the  end  of  the  tail.  On  the 
12th  of  April,  its  length  was  sixty  degrees,  and  ap- 
peared again  undivided,  the  secondary  train,  which 
was  not  as  bright  as  the  primary,  having  apparently 
vanished.  On  the  14th  of  April,  the  tail  extended 
from  the  horizon  to  the  zenith,  and  the  appearance 
of  the  comet  was  extremely  grand.  After  this  date 
8 


86  TREATISE   ox    COMETS. 

the  length  of  the  tail  decreased  rapidly,  and  on  the 
28th  of  April  the  comet  was  seen  for  the  last  time, 
with  a  train  only  three  degrees  long.  Some  astrono- 
mers have  supposed  this  comet  to  have  been  a  former 
appearance  of  a  comet  which  appeared  in  1682,  and 
which  is  now  known  as  Halley's  comet. 

One  of  the  largest  and  most  remarkable  comets 
on  record  made  its  appearance  in  the  year  895.  It 
was  seen  in  China  on  the  25th  of  June,  in  that  year, 
and  the  tail  or  brush  is  said  to  have  been  one  hun- 
dred degrees  in  length.  When  the  comet  first  be- 
came visible  it  was  in  the  Great  Bear,  but  it  subse- 
quently moved  towards  the  constellation  Hercules. 
The  brilliancy  increased  very  rapidly,  and,  accord- 
ing to  the  Chinese  historians,  it  actually  attained  the 
enormous  and  unexampled  length  of  two  hundred 
degrees.  This  account  is  undoubtedly  somewhat 
exaggerated,  but  it  is  sufficiently  certain  that  the 
length  of  the  train  was  unsurpassed  by  any  other 
which  had  ever  before  been  recorded.  Another 
large  comet  appeared  in  the  autumn  of  the  year 
975.  In  China  it  was  seen  on  the  3d  of  August,  in 
the  constellation  Hydra,  at  which  time  the  tail  was 
forty  degrees  in  length.  The  comet  was  visible 
during  a  period  of  eighty-three  days,  and  passed 
through  the  constellations  Cancer,  Gemini,  Taurus, 
and  Aries,  and  a  part  of  Pegasus  and  Andromeda. 
The  course  pursued  by  the  comet  was  such  that 


DESCRIPTION   OF   COMETS.  87 

Pingre  supposes  it  to  be  identical  with  the  great 
comet  which  appeared  in  1264  and  in  1556.  By 
supposing  that  the  comet  passed  nearest  the  sun  a 
few  days  before  the  end  of  July,  975,  it  may  be 
shown  that  it  would  have  been  in  conjunction  with 
the  sun  on  or  about  the  3d  of  August,  but  that  its 
north  latitude  being  considerable,  it  would  rise 
before  the  sun,  and  might  exhibit  all  the  phenomena 
which  have  been  recorded. 

In  April,  1066,  a  comet  of  great  brilliancy  was 
seen  both  in  Europe  and  in  China.  It  was  supposed 
by  European  historians  to  be  the  forerunner  of  the 
conquest  of  England,  by  William,  Duke  of  Nor- 
mandy. The  course  of  the  comet  seems  to  render 
it  probable  that  it  is  "a  previous  return  of  the  comet 
of  1677.  Another  comet  was  seen  for  a  very  short 
time,  in  October,  1097.  It  was  visible  in  Europe 
only  fifteen  days,  and  is  remarkable  on  account  of 
having  two  separate  trains,  which  were  directed  to- 
ward the  east  and  south-east  respectively.  In  China, 
on  October  6th  and  9th,  its  tail  was  noted  as  being 
respectively  thirty  and  fifty  degrees  in  length.  On 
the  6th  of  October  it  was  seen  in  the  constellation 
Libra,  and  on  the  16th  of  the  same  month  it  had 
moved  north  as  far  as  the  head  of  Hercules.  It 
ceased  to  be  visible  in  China  on  the  25th  of  October. 
A  large  and  brilliant  comet  was  also  seen  in  Pales- 
tine and  in  China  early  in  February,  1106.  Its  tail 


88  TREATISE   ON    COMETS. 

was  sixty  degrees  in  length,  and  was  similar  in  color 
to  the  whiteness  of  snow.  The  comet  was  visible 
during  a  period  of  fifty  days. 

In  the  year  1264,  a  great  and  celebrated  comet 
made  its  appearance,  and  its  magnitude  was  such 
that  it  is  mentioned  by  all  the  historians  of  that  day ; 
and  it  is  also  distinctly  stated  that  no  one  living  at 
the  time  of  its  appearance  had  ever  seen  one  which 
could  be  compared  with  it.  It  was  first  seen  about 
the  1st  of  July,  and  attained  its  maximum  brilliancy 
in  the  latter  part  of  August  and  in  the  beginning 
of  September.  The  tail  was  one  hundred  degrees 
in  length,  and  appeared  curved  in  the  form  of  a 
sabre.  The  train  was  visible  in  the  east  early  in  the 
evening,  although  the  nucleus  did  not  appear  above 
the  eastern  horizon  until  near  morning.  The  comet 
was  last  seen  on  the  evening  of  the  3d  of  October, 
the  date  of  the  death  of  Pope  Urban  IV.,  of  which 
event  it  was  considered  the  precursor.  The  course 
pursued  by  the  comet  was  such  that,  in  more  modern 
times,  it  has  been  conjectured  that  this  body  is 
identical  with  the  great  comets  which  appeared  in 
975  and  in  1556. 

We  have  here,  and  also  in  the  preceding  pages, 
alluded  to  the  supposed  identity  of  comets  which 
have  appeared  at  different  epochs.  These  allusions 
may  seem  premature,  from  the  fact  that,  thus  far,  no 
account  has  been  given  of  the  nature  and  forms  of 


DESCRIPTION   OF   COMETS.  89 

the  paths  pursued  by  the  comets  in  their  motions 
through  space.  For  reasons  which  will  presently 
be  made  obvious,  it  has  seemed  expedient  to  defer 
a  complete  exposition  of  the  theory  of  the  motions 
of  the  comets,  until  we  come  to  treat  of  those  comets 
which  are  definitely  known  to  have  fixed  periods  of 
revolution,  and  which  have  been  certainly  identified 
at  two  or  more  different  visits  to  our  vicinity. 
Under  such  circumstances  it  may  be  advisable  to 
state,  at  once,  in  reference  to  the  forms  of  the 
cometary  orbits,  what  will  sufl3.ce  for  our  present 
purpose,  reserving  for  the  proper  connection  a  de- 
tailed explanation  of  the  theory  of  the  motions  of 
these  chaotic  worlds. 

The  comets  are  known  to  move  in  regular  orbits 
in  three  different  kinds  of  curves,  in  obedience  to 
the  law  of  universal  gravitation.  These  curves  are 
the  ellipse,  parabola,  and  hyperbola,  the  nature  of 
which  will  be  subsequently  explained.  Those  which 
move  in  ellipses  will  return  regularly  to  the  sun  at 
stated  intervals,  while  those  which  move  in  para- 
bolas and  hyperbolas  visit  our  system  but  once,  and 
thence  pass  on  to  visit  other  suns  and  systems.  Of 
all  the  comets  which  have  been  discovered,  a  ma- 
jority are  fo-und  to  move  in  these  curves,  and  the 
number  which  move  in  elliptic  orbits  whose  dimen- 
sions are  confined  within  the  probable  bounds  of  the 
solar  system,  is  much  more  limited.  But  a  small 


90  TREATISE   ON    COMETS. 

number,  comparatively,  have  been  observed  at  more 
than  one  appearance,  and  these  we  propose  to  con- 
sider as  a  separate  and  distinct  class.  There  are 
many  others  which  the  delicate  refinements  of 
modern  astronomy  have  shown  to  move  in  orbits 
whose  ellipticity  is  clearly  indicated,  but  in  which 
the  deviation  from  either  the  hyperbola  or  parabola 
is  so  uncertain  that  it  is  impossible  to  predict, 
within  very  narrow  limits,  the  precise  date  of  their 
return.  The  comet  of  1264  is  one  of  these ;  and 
although  the  elements  which  determine  the  form  and 
position  of  its  orbit  in  space,  are  found  to  bear  a  close 
resemblance  to  those  which  have  been  obtained  for 
the  comets  of  975  and  1556,  yet  we  are  unable  to 
venture  a  prediction  that  these  comets  are  identical, 
and  that  another  appearance  may  be  expected  after 
the  lapse  of  about  300  years  from  the  date  of  its  last 
visit  to  the  sun.  The  point  of  the  orbit  which  is 
nearest  the  sun  is  called  the  perihelion ;  the  point 
which  is  most  remote  is  called  the  aphelion ;  and 
the  nature  of  these  curves  is  such,  that  for  a  consi- 
derable distance  on  either  side  of  the  perihelion 
they  nearly  coincide.  This,  together  with  the  un- 
certainty of  the  observations,  and  the  fact  that  the 
comet  is  visible  from  the  earth  only  when  near  the 
sun,  sometimes  renders  it  impossible  to  decide  in 
regard  to  the  exact  form  of  the  orbit.  "With  these 
explanatory  remarks,  we  will  continue  the  descrip- 


DESCRIPTION   OF   COMETS.  91 

tioD  of  the  most  remarkable  comets  which  have 
appeared  subsequent  to  the  one  we  have  just 
noticed. 

In  the  year  1402  a  very  large  and  brilliant  comet 
made  its  appearance.  It  was  seen  at  different 
places  on  the  8th  of  February,  and  was  so  brilliant 
that  it  could  be  seen  at  mid-day.  It  disappeared 
early  in  March.  Another  comet  of  extraordinary 
magnitude  made  its  appearance  in  1456.  It  is  rep- 
resented by  historians  as  being  grand  and  terrible, 
with  a  tail  more  than  sixty  degrees  in  length.  It 
was  visible  during  the  month  of  June,  and  was  very 
remarkable  on  account  of  the  rapid  variations  in 
the  size  of  the  train,  which  at  one  time  did  not 
exceed  seven  degrees  in  length.  The  first  appear- 
ance of  the  comet  was  so  sudden  and  terrific,  that  a 
belief  became  very  prevalent,  among  all  classes, 
that  it  would  destroy  the  earth,  and  that  the  Day 
of  Judgment  was  at  hand.  This  is  the  comet  which 
we  have  already  noticed  as  having  created  a  most 
profound  sensation  at  Rome,  since  it  was  regarded 
as  having  presaged  the  rapid  success  of  the  Turks, 
who  were  then  engaged  in  subjugating  the  nations 
of  Europe.  The  fact  that  the  followers  of  Moham- 
med had  already  crossed  the  Hellespont,  and  that 
no  force  had  been  able  to  check  their  progress, 
together  with  the  great  apprehension  which  the 
appearance  of  the  comet  had  already  excited,  seemed 


92  T  11  E  A  T  I  S  E     0  N     C  0  M  E  T  S  . 

to  add  to  the  general  gloom  which  pervaded  all 
classes  of  society ;  and  it  was  under  these  circum- 
stances that  Pope  Calixtus  II.  ordered  prayers  to  be 
offered  daily  for  preservation  from  the  baneful  influ- 
ences of  the  comet,  the  devil,  and  the  Turks,  and 
issued  a  bull  in  which  all  three  were  equally  ana- 
thematized. This  comet  is  now  recognized  as  a 
previous  return  of  the  comet  of  1682,  known  as 
Halley's  comet;  and  its  great  brilliancy  resulted 
from  the  favorable  circumstances,  in  this  respect, 
of  its  position  with  reference  to  the  earth  and  sun. 

In  February,  1490,  a  comet  appeared  in  Europe 
which  had  a  small  nucleus,  but  a  train  extending 
to  an  enormous  distance,  and  remarkable  for  its 
phosphorescent  appearance.  It  was  visible  at  Bou- 
logne about  the  middle  of  February,  and  was  pro- 
bably not  seen  in  China  and  Japan.  A  comet  is 
recorded  in  the  Chinese  annals  in  the  year  1491,  but 
there  is  hardly  any  reason  to  believe  it  to  have  been 
identical  with  this. 

The  comet  of  1456  again  made  its  appearance  in 
July  and  August,  1531,  but  was  far  from  the  earth 
and  sun,  and  consequently  not  very  brilliant.  The 
year  following,  1532,  a  comet  was  seen  in  China  and 
also  in  Europe,  which  has  been  supposed  to  be  iden- 
tical with  one  which  appeared  in  1661.  It  was 
visible  in  the  morning  before  sunrise,  and  the  nu- 
cleus is  described  as  being  apparently  three  times 


GREAT   Co M E T   o v   1  o o 6 .  93 

larger  than  Jupiter.  It  was  visible  in  Europe  about 
seventy  days,  and  in  China  one  hundred  and  fifteen 
days.  The  train  varied  in  length  from  one  to  ten 
degrees,  and  was  very  bright. 

In  1556  another  comet  of  prodigious  magnitude 
made  its  appearance.  It  began  to  be  generally 
visible  about  the  end  of  February,  at  which  time  it 
is  said  to  have  been  more  than  half  as  large  as  the 
full  moon.  Its  tail,  however,  was  short  and  vari- 
able, and  for  this  reason  it  was  described  as  exhi- 
biting a  movement  like  that  of  a  flame,  or  a  torch 
disturbed  by  the  wind.  The  greatest  length  of  the 
tail  did  not  exceed  four  degrees,  and  its  color  was 
ruddy,  not  unlike  that  of  the  planet  Mars.  It 
moved  rapidly  north,- and  on  the  12th  of  March  was 
distant  from  the  earth  only  eight  millions  of  miles. 
The  comet  disappeared  in  the  sun's  rays  on  the  23d 
of  April,  in  the  southern  part  of  the  constellation 
Cassiopeia.  It  created  considerable  excitement  in 
Europe,  and  is  said  to  have  been  the  immediate 
cause  of  the  abdication  of  Charles  V.,  Emperor  of 
Germany. 

The  orbit  of  this  comet  was  computed  by  Dr. 
Halley ;  but  since  the  observations  on  which  they 
were  based  were  uncertain,  they  have  not  been  con- 
sidered very  accurate.  They  were  sufficient,  how- 
ever, to  indicate  the  probable  identity  of  this  comet 
with  those  which  had  been  observed  in  975  and 


94  TREATISE   ON    COMETS. 

1264.  He  was  therefore  led  to  conjecture  that  the 
comet  had  a  period  of  revolution  around  the  sun  of 
about  292  years,  and  that  it  might  he  expected  to 
reappear  in  1848.  A  few  years  later  Pingre  col- 
lected together  all  the  observations  of  the  comets 
of  9T5  and  1264,  and  more  especially  the  latter, 
which  could  be  found  in  both  the  European  and 
Chinese  annals,  and  from  a  complete  investigation 
of  the  elements  of  the  orbits  of  the  comets  of  1264 
and  1556,  he  found  that  the  courses  of  both  comets, 
at  the  date  of  their  appearance,  could  be  very  satis- 
factorily represented  by  the  same  elements.  He 
therefore  confirmed  the  hypothesis  of  Halley,  and 
concurred  in  the  opinion  that  the  comet  might  be 
expected  again  in  1848.  Nothing  further  was  at- 
tempted until  the  time  had  nearly  elapsed  at  which 
it  was  to  reappear.  Between  the  years  1843  and 
1847  the  whole  subject  was  investigated  anew  by 
Mr.  Hind,  of  London,  arid  M.  Bomme,  of  Middle- 
burg,  in  the  Netherlands.  Mr.  Hind  found  that  the 
elements  were  such  as  to  confirm  their  supposed 
identity ;  and  by  taking  into  account  the  planetary 
perturbations,  he  concluded  that  the  comet  would 
appear  about  the  year  1858. 

The  computations  of  Bomme  were  more  extended, 
and  he  found  that  with  Halley 's  elements,  at  the 
time  the  comet  was  visible  in  1264,  it  was  moving 
in  an  ellipse,  with  a  periodic  time  of  112,469  days, 


GREAT  COMET  OF  1556.      95 

or  about  308  years ;  but  tbat  the  disturbances  pro- 
duced by  the  attraction  of  the  planets  would  accele- 
rate its  motion,  and  shorten  its  period  5903  days,  so 
that  it  passed  its  perihelion  in  April,  1556.  He 
found  that  at  this  return  it  was  describing  an  elliptic 
orbit  which  gave  a  period  of  112,943  days;  but  that 
the  next  revolution  would  occupy  a  period  of  111,146 
days,  bringing  the  comet  to  its  perihelion  on  the 
22d  of  August,  1860. 

With  Hind's  elements  it  was  found  that,  in  1264, 
the  ellipse  described  by  the  comet  corresponded  to 
a  period  of  110,644  days,  or  nearly  303  years ;  and 
that  the  attraction  of  the  planets  would  shorten  its 
period  a  little  more  than  eleven  years,  thus  bringing 
the  comet  to  its  perihelion  in  1556.  The  same  ele- 
ments indicated  that  at  the  time  it  was  visible  in 
1556,  its  mean  motion  corresponded  to  a  period  of 
308  years,  and  that  the  period  of  its  next  return 
would  be  expedited  by  planetary  disturbances  3828 
days,  or  ten  and  a  half  years.  The  comet  would, 
therefore,  arrive  at  its  perihelion  early  in  August, 
1858.  Thus  it  appears  that  there  is  an  uncertainty 
of  at  least  two  years  in  the  time  of  its  next  appear- 
ance ;  and  since  the  interval  included  between  the 
results  of  Hind  and  Bomme  has  now  elapsed,  we 
must  conclude  that  those  of  the  latter  are  most  accu- 
rate, although  themselves  perhaps  very  far  from  the 
truth. 


96  TREATISE   ON    COMETS. 

The  important  circumstance  of  an  apparent  iden- 
tity between  the  comet  of  975  and  those  of  1264 
and  1556,  has  already  been  noticed ;  and  it  is  found 
that,  assuming  the  identity,  and  adopting  Hind's 
elements,  the  path  of  the  former  may  be  very  closely 
represented.  This  question,  however,  has  been  ela- 
borately discussed  by  M.  Hoek,  of  Leiden,  and  he 
concludes  that  there  is  no  reason  to  regard  the 
comets  as  identical.  He  finds  that  the  path  of  the 
comet  cannot  be  satisfactorily  represented  by  the 
elements  of  either  the  comet  of  1264  or  1556,  each 
of  which  he  has  investigated  anew.  He  finds  also 
that  there  exist  great  discrepancies  even  between 
the  orbits  of  the  latter  comets,  and  announces  that 
their  identity  is  extremely  doubtful.  It  appears, 
therefore,  that  it  is  barely  possible  that  the  comet  of 
1556  may  shortly  appear,  although  no  such  predic- 
tion can  be  entitled  to  any  particular  confidence. 
Should  the  comet  really  make  its  appearance,  its 
future  movements  would  no  longer  be  involved  in 
doubt,  but  its  next  and  succeeding  returns  could  be 
predicted  with  the  utmost  precision. 

The  next  comet  of  any  considerable  magnitude, 
compared  with  those  which  we  have  already  de- 
scribed, made  its  appearance  in  1590.  It  was  dis- 
covered by  Tycho  Brahe,  on  the  5th  of  March,  and 
observed  until  the  16th  of  the  same  month.  It  is 
noted  as  being  of  a  medium  magnitude,  but  had  a 


COMETS   FROM   1590   TO   1668.          97 

great  tail,  which  extended  as  far  as  the  zenith,  the 
nucleus  being  near  the  horizon.  On  the  day  of  its 
discovery,  the  comet  appeared  as  bright  as  a  star  of 
the  second  magnitude ;  and  shortly  afterwards,  on 
the  same  night,  it  was  equal  in  brilliancy  to  a  star 
of  the  first  magnitude.  Another  large  comet  ap- 
peared in  1618.  It  was  discovered  in  Silesia  on  the 
10th  of  November,  and  at  Rome  on  the  same  even- 
ing. It  was  also  observed  by  the  Spanish  Ambassa- 
dor at  Ispahan,  in  Persia,  for  fifteen  days,  com- 
mencing with  the  date  of  its  discovery  in  Europe ; 
and  presented  a  magnificent  appearance  in  the  east- 
ern sky,  about  two  hours  before  sunrise.  The  length 
of  the  tail  was  about  60  degrees. 

In  December,  1652,  a  comet  was  seen  in  Europe, 
which  was  of  a  pale  and  livid  color,  and  almost 
equalled  the  moon  in  magnitude  and  brilliancy.  No 
mention  is  made  of  the  length  of  its  tail.  Another 
comet  of  considerable  size  was  seen  in  Spain,  on  the 
17th  of  November,  1664.  It  was  subsequently  ob- 
served in  France  and  Germany ;  and  was  last  seen 
by  Helvetius,  at  Dantzic,  on  the  18th  of  February, 
1666.  At  its  first  appearance,  it  was  noted  as  large 
as  a  star  of  the  first  magnitude,  but  not  so  bright. 
The  length  of  the  tail  varied  from  five  to  ten  de- 
grees. Four  years  later,  1668,  the  tail  of  a  comet 
was  seen  above  the  southern  horizon,  at  different 
places  in  Italy.  The  nucleus  was  not  visible,  but 
9 


98  TREATISE   ON   COMETS. 

the  comet  was  observed  in  the  southern  hemisphere. 
This  comet  has  been  supposed  to  be  identical  with 
the  great  comet  which  appeared  in  1843,  and  is  re- 
markable for  its  near  approach  to  the  sun. 

One  of  the  most  remarkable  comets  on  record 
made  its  appearance  toward  the  close  of  the  year 
1680,  and  was  carefully  observed  by  the  astronomers 
of  Europe.  It  is  said  to  have  appeared  to  descend, 
as  it  were,  from  the  distant  regions  of  space  with  a 
prodigious  velocity,  almost  perpendicular  to  the  sun, 
and  to  have  ascended  again  in  the  same  manner, 
with  a  velocity  retarded  as  it  had  before  been  acce- 
lerated. The  comet  was  discovered  at  Coburg,  in 
Saxony,  on  the  4th  of  November,  1680 ;  but  the 
name  of  the  discoverer  is  unknown.  It  was  also 
discovered  independently  by  Godfrey  Kirch,  on  the 
14th  of  November,  while  about  to  observe  the  moon 
and  Mars.  It  was  very  carefully  observed  at  Paris 
by  Oassini,  during  the  entire  period  of  its  visibility. 
It  was  observed  by  Flamstead,  at  Greenwich,  until 
the  15th  of  February,  1681.  The  observations  of 
Cassini,  Flamstead,  and  others,  enabled  Newton  to 
trace  out  the  form  of  its  orbit,  and  its  position  in 
space ;  and  he  found  that  the  observations  could  be 
satisfactorily  represented  by  supposing  the  orbit  to 
be  a  parabola,  with  the  sun  in  the  focus.  At  its 
perihelion,  he  found  that  the  distance  of  the  comet 
from  the  surface  of  the  sun  did  not  exceed  one-sixth 


GREAT   COMET   OF   1680.  99 

of  the  sun's  diameter,  or  about  147,000  miles.  The 
velocity  of  the  comet,  wheu  nearest  the  sun,  must 
therefore  have  been  880,000  miles  an  hour. 

A  more  complete  determination  of  the  elements 
of  the  orbit  of  this  comet  was  made  by  Dr.  Halley, 
who  found  that  the  orbit  was  sensibly  an  ellipse, 
with  the  sun  in  one  of  the  foci ;  and  that  the  peri- 
helion distance  was  only  590,000  miles  from  the 
sun's  centre,  while  its  aphelion  distance  was  at  least 
13,000,000,000,  or  thirteen  thousand  millions  of 
miles.  He  determined  also  that,  in  passing  the  point 
of  intersection  of  its  orbit  with  the  plane  of  the 
earth's  orbit,  in  moving  from  the  north  to  the  south 
side  of  the  latter,  or  in  passing  through  what  is 
called  the  descending  node,  the  comet  was  distant 
from  the  earth  only  440,000  miles.  It  passed  through 
this  point  on  the  22d  of  November;  and  it  was  re- 
marked by  Halley  that,  had  the  earth  been  then  in 
that  part  of  its  orbit  nearest  the  descending  node 
of  the  comet,  their  mutual  gravitation  would  have 
caused  a  change  in  the  plane  of  the  earth's  orbit, 
and  in  the  length  of  our  year;  and  if  so  large  a 
body,  with  so  rapid  a  motion,  were  to  strike  the 
earth,  the  shock  might  reduce  this  beautiful  struc- 
ture to  its  original  chaos. 

Whistoii  supposed  this  comet  to  be  identical  with 
the  comets  which  appeared  in  the  years  1106,  531, 
and  43,  B.  C. ;  and  under  this  supposition  he  com- 


100  TREATISE   ON    COMETS. 

putes  backward,  and  attempts  to  explain  the  phe- 
nomena of  the  Mosaic  deluge  by  a  near  approach 
of  the  comet  to  the  earth,  as  already  explained. 
The  computations  of  Euler,  Pingre,  and  Encke,  have 
fully  confirmed  the  supposed  ellipticity  of  its  orbit,  but 
indicate  that  its  period  of  revolution  must  be  vastly 
greater  than  575  years ;  and  that,  consequently,  the 
speculations  of  "Whiston,  at  least  so  far  as  this  comet 
is  concerned,  must  be  regarded  as  extremely  absurd. 
This  comet  was  remarkable  for  its  near  approach  to 
the  earth  and  sun,  and  also  for  the  brilliancy  of  its 
light  and  the  length  of  its  tail,  which  extended  over 
half  the  vault  of  heaven,  or  from  the  zenith  to  the 
horizon. 

In  the  month  of  August,  1686,  a  comet  was  visible 
at  Para,  in  Brazil,  whose  nucleus  was  as  bright  as  a 
star  of  the  first  magnitude.  The  tail  was  eighteen 
degrees  in  length,  and  is  described  as  having  pre- 
sented a  very  beautiful  appearance.  Three  years 
later  a  comet  of  uncommon  splendor  was  seen  in 
China  and  at  different  places  in  the  southern  hemi- 
sphere. It  was  first  discovered  at  Pekin  on  the  llth 
of  December,  1689,  but  was  not  visible  in  Europe. 
On  the  evening  of  its  discovery  the  nucleus  was  very 
brilliant,  and  the  tail  was  about  twelve  degrees  in 
length.  In  the  southern  hemisphere  the  comet  was 
much  more  favorably  situated  for  observation,  and 
consequently  its  general  appearance  was  correspond- 


COMET   OF   1744.  101 

iugly  improved.  It  was  equal  in  brilliancy  to  the 
bright  stars  of  the  first  magnitude,  and  the  greatest 
length  of  the  tail  was  60  degrees.  Another  comet, 
similar  in  size  and  appearance  to  that  of  1686,  just 
described,  was  seen  in  Brazil  on  the  28th  of  October, 
1695.  The  length  of  the  tail  was  18  degrees,  and 
the  nucleus  was  remarked  to  be  at  first  almost  ob- 
scured by  the  atmosphere  or  coma  by  which  it  was 
surrounded,  and  was  even,  at  intervals,  scarcely  dis- 
tinguishable. 

The  first  part  of  the  eighteenth  century  was  not 
remarkable  either  for  the  number  or  brilliancy  of 
the  comets  which  appeared.  There  were,  however, 
several  of  sufficient  size  and  brilliancy  to  be  visible 
to  the  naked  eye,  but  not  such  as  to  present  a  very 
striking  appearance.  It  was  not,  therefore,  till  1744 
that  a  comet  was  visible  which  deserves  a  notice  in 
this  connection.  This  comet  was  discovered  at 
Harlem  on  the  9th  of  December,  1743,  and  was  ob- 
served at  several  observatories,  until  about  the  time 
of  its  perihelion  passage,  which  took  place  on  the 
evening  of  the  1st  of  March,  1744.  It  was  one  of 
the  largest  and  most  beautiful  comets  which  had 
appeared  ,since  the  famous  one  of  1680,  and  there- 
fore excited  no  small  degree  of  interest  throughout 
the  civilized  world.  The~  nucleus  was  very  bright, 
and  although  the  tail  was  not  remarkable  for  its 
length,  yet  it  was  noticed  by  Cassini,  early  in  Feb- 
9* 


102  TREATISE   ON    COMETS. 

ruary,  that  the  head  was  separated  into  two  distinct 
parts,  and  the  train  exhibited  decided  symptoms  of 
a  corresponding  appearance.  The  tail  was,  indeed, 
shortly  after  this  observation  of  Cassini,  observed 
to  be  divided  into  two  branches.  At  one  time  sub- 
sequently, and  just  before  its  passage  through  the 
perihelion  of  its  orbit,  it  is  said  to  have  had  six 
separate  and  distinct  tails,  the  lower  one  relatively 
to  the  horizon  being  greatly  curved  near  the  ex- 
tremity. The  accompanying  diagram  represents  the 
appearance  of  the  comet  at  this  time.  The  remark- 
able phenomenon,  however,  of  six  distinct  tails  was 
of  only  a  few  days'  duration,  and  the  comet  appeared 
again  as  before,  having  but  two  trains.  This  comet 
approached  the  sun  to  within  twenty  millions  of 
miles,  and  the  elements  of  its  orbit  which  have  been 
computed  do  not  seem  to  indicate  that  it  will  ever 
reappear  to  human  vision. 

The  year  1748  was  remarkable  for  the  appearance 
of  two  bright  comets  at  nearly  the  same  time.  The 
first  was  discovered  at  Paris  about  the  end  of  April 
of  that  year,  and  was  also  observed  in  South  Ame- 
rica, and  in  China.  It  is  described  as  having  been 
a  fine  object,  easily  seen  with  the  naked  eye,  and 
having  a  tail  twenty  degrees  in  length.  It  was  last 
observed  by  Maraldi,  at  Paris,  on  the  30th  of  June. 
The  second  was  discovered  at  Harlem,  on  the  19th 
of  May,  in  a  different  part  of  the  heavens  from  that 


GREAT  COMET   OF   1769.  103 

in  which  the  one  just  described  was  seen.  Its  nucleus 
was  brighter  than  the  preceding,  but  there  was  no 
appearance  of  a  tail.  From  this  date  until  1769,  if 
we  except  a  reappearance  of  the  comet  of  1682, 
known  as  Halley's  comet,  which  will  be  described 
hereafter,  no  comet  of  considerable  size  was  ob- 
served. 

The  great  comet  of  1769  was  discovered  by  Mes- 
sier, at  Paris,  on  the  8th  of  August,  and  was  subse- 
quently observed  throughout  Europe.  It  was  ob- 
served^ by  La  Nux  at  the  Isle  of  Bourbon,  in  the 
Indian  Ocean,  from  the  26th  of  August  to  the  26th 
of  September ;  and  all  the  various  changes  which 
took  place  in  the  form  and  size  of  both  the  nucleus 
and  the  tail  were  carefully  recorded.  On  the  llth 
of  September  the  length  of  the  tail  was  found  to  be 
97  degrees,  while  on  the  28th  of  August  it  did  not 
much  exceed  15  degrees.  On  the  9th  of  September 
the  length  of  the  tail  was  variously  estimated  from 
43  to  75  degrees.  It  passed  nearest  the  sun  on  the 
7th  of  October,  about  midnight,  at  a  distance  of 
eleven  millions  of  miles.  It  was  nearest  the  earth 
about  the  middle  of  September,  and  was  distant 
about  ten  millions  of  miles.  It  was  last  seen  on  the 
1st  of  December,  having  been  visible  more  than 
three  months.  At  the  time  of  the  appearance  of  this 
comet,  the  methods  of  determining  the  form  and 
position  of  the  orbits  of  these  bodies  in  space  by 


104  TREATISE   ON    COMETS. 

means  of  observations  made  during  a  few  days,  had 
been  successfully  investigated ;  and  since  the  astrono- 
mers were  enabled  to  predict  from  day  to  day,  and 
even  for  several  weeks  in  advance,  the  variety  and 
grandeur  of  the  phenomena  which  it  would  present, 
even  to  the  unassisted  eye,  it  very  naturally  excited 
a  lively  interest  among  all  classes  of  society.  The 
tail  was  not  actually  as  long  as  had  been  exhibited 
in  the  case  of  many  other  comets  which  had  ap- 
peared ;  but  the  near  approach  of  the  comet  to  both 
the  earth  and  sun,  and  at  very  nearly  the  same  time, 
gave  it  a  most  magnificent  appearance,  and  caused 
it  to  appear  even  greater  than  it  really  was.  The 
researches  of  modern  astronomers  seem  to  indicate 
that  the  comet  will  return  again  to  our  system  after 
the  lapse  of  a  few  thousand  years,  but  fail  to  indicate 
within  any  very  close  limits  the  exact  period  of  its 
return. 

During  the  last  half  of  the  eighteenth  century, 
no  less  than  forty-five  comets  were  observed ;  but, 
with  the  exception  of  Halley's  comet,  and  that  just 
described,  they  were  not  remarkable  for  .their  bril- 
liancy. Some  of  them  were  bright  enough  to  be 
distinguished  by  the  naked  eye,  and  sometimes  ex- 
hibited a  faint  train  a  few  degrees  in  length ;  but 
the  great  majority  were  round  and  ill-defined  nebu- 
losities, with  a  slight  condensation  of  light  at  the 
centre. 


GREAT  COMET  OF  1807.      105 

The  next  comet  of  considerable  size  was  disco- 
vered by  Pons,  at  Marseilles,  on  the  20th  of  Sep- 
tember, 1807.  This  is  the  date  of  the  first  authentic 
observation,  although  it  seems  to  have  been  seen 
eight  days  previously  by  an  Augustine  monk  in 
Italy.  This  was  the  finest  comet  which  had  ap- 
peared since  that  of  1769.  On  the  30th  of  September 
the  nucleus  was  nearly  as  bright  as  a  star  of  the 
second  magnitude,  and  the  tail  was  distinctly  visible. 
On  the  8th  of  October  it  was  seen  in  the  north-west, 
soon  after  sunset,  and  not  far  distant  from  Arcturus, 
which  was  then  only  a  few  degrees  above  the  hori- 
zon. It  is  described  as  presenting  to  the  naked  eye 
the  appearance  of  a  dim  nebulous  star  of  the  second 
magnitude,  with  a  beam  of  light  on  one  side  of  it. 
Through  a  telescope  its  tail  presented  a  brilliant 
appearance,  and  was  more  than  a  degree  in  length. 
The  coma,  and  even  the  nucleus,  were  remarkably 
ill-defined.  On  the  7th  of  November  the  tail  was 
seen  divided  into  two  parts ;  and,  a  few  days  later, 
the  comet  ceased  to  be  visible  to  the  naked  eye.  It 
was  last  observed  on  the  27th  of  March,  1808,  at 
which  date  it  was  a  very  faint  telescopic  object. 

The  orbit  of  this  comet  has  been  very  carefully 
determined  by  Bessel,  and  he  finds  that  it  is  an 
ellipse,  corresponding  to  a  period  of  revolution  of 
about  1500  years.  The  comet  passed  the  perihelion 
of  its  orbit  in  the  forenoon  of  the  19th  of  Septem- 


106  TREATISE   ON   COMETS. 

her,  at  a  distance  from  the  sun  of  about  62,000,000 
miles.  The  aphelion  distance  is  about  eleven  thou- 
sand millions  of  miles.  The  diameter  of  the  nucleus 
was  about  4600  miles,  or  about  the  size  of  the  planet 
Mars,  and  seemed  to  be  of  considerable  density. 
The  diameter  of  the  coma  or  envelope  was  estimated 
at  120,000  miles,  but  appeared  to  vary  at  different 
dates ;  and  the  velocity  of  the  comet  in  its  orbit, 
when  nearest  the  sun,  was  considerably  more  than 
two  millions  of  miles  daily.  The  tail,  as  already  re- 
marked, was  divided  into  two  separate  branches. 
The  north  side,  however,  was  much  brighter  and 
better  defined  than  the  other,  and  was  also  invariably 
convex,  while  the  other  was  concave.  The  most 
remarkable  feature  about  the  tail  was  the  rapid 
variations  which  took  place  in  its  length  and  bril- 
liancy. It  was  seen  at  times  to  exhibit  coruscations 
or  flashes  of  light,  not  unlike  what  is  exhibited  in 
the  case  of  the  aurora  borealis.  It  is  asserted  that 
in  less  than  one  second  of  time,  streamers  might  be 
seen  to  shoot  out  from  the  body  of  the  tail,  and 
generally  from  the  vicinity  of  the  nucleus,  to  a  dis- 
tance of  two  and  a  half  degrees,  and  that  they  as 
rapidly  disappeared  and  issued  out  again.  These 
coruscations  may  perhaps  have  been  much  less 
frequent  than  what  is  here  stated,  since  the  accounts 
given  are  very  probably  exaggerated ;  but,  as  will 
be  evident  in  the  case  of  comets  which  appeared 


GREAT   COMET   OF   1811.  107 

subsequently,  there  can  be  no  doubt  but  that 
streamers  were  observed  to  reach  out  even  as  far  as 
the  whole  length  of  the  tail,  or  to  a  distance  of 
nearly  five  millions  of  miles.  The  light  of  these 
streamers  was  also  described  as  being  sometimes 
whiter  and  clearer  at  the  end  than  at  the  base. 

We  come  now  to  consider  the  case  of  the  great 
comet  of  1811,  which  was  discovered  by  Flaugergues, 
at  Vivieres,  on  the  26th  of  March  of  that  year. 
"When  first  discovered  it  was  a  faint  nebulous  object, 
with  a  slow  motion  as  seen  from  the  earth,  and  far 
distant  from  both  the  earth  and  sun.  It  did  not 
exhibit  any  signs  of  a  tail,  but  presented  simply  a 
slight  condensation  of  light  at  the  centre.  It  con- 
tinued visible  until  the  10th  of  June,  when  it  was 
lost  in  the  approaching  twilight.  It  had,  however, 
been  already  extensively  observed  by  the  astron- 
omers of  Europe  ;  and  from  the  observations  it  was 
found,  by  computing  its  orbit,  that  it  would  not 
pass  its  perihelion  until  about  the  12th  of  September, 
and  that  it  would  again  emerge  from  the  sun's  rays 
early  in  August.  It  was  found,  also,  that  it  was 
rapidly  approaching  both  the  earth  and  sun,  and 
that  it  would  certainly  present  a  magnificent  appear- 
ance during  the  entire  months  of  September  and 
October.  These  predictions  were  indeed  realized 
to  the  very  fullest  extent.  The  comet  was  again 
detected  on  the  20th  of  August,  and  observed  almost 


108  TREATISE   ON    COMETS. 

without  interruption  till  the  llth  of  January,  1812, 
when  it  was  again  lost  in  the  twilight.  It  was, 
however,  detected  the  third  time  by  Wisniewsky, 
in  Neu-Tscherkask,  on  the  31st  of  July,  1812,  and 
observed  for  the  last  time  on  the  17th  of  August  of 
the  same  year,  having  been  visible  for  a  period  of 
nearly  seventeen  months.  It  was  visible  to  the 
naked  eye  during  a  period  of  more  than  three 
months ;  and,  on  account  of  its  immense  size  and 
unexampled  brilliancy,  excited  a  most  profound 
sensation.  To  astronomers  it  was  especially  inte- 
resting, on  account  of  the  facilities  which  its  long 
visibility  afforded  for  the  determination  of  the  ele- 
ments of  its  orbit,  with  the  very  greatest  precision  ; 
and,  also,  for  the  opportunity  of  observing  its  aspect 
and  its  physical  constitution.  The  great  perfection 
to  which  theoretical  and  practical  astronomy  had 
been  carried  at  the  date  of  its  appearance,  and  the 
great  number  of  observers,  provided  with  superior 
instruments,  rendered  it  possible  to  investigate  with 
great  facility  all  the  various  questions  which  might 
arise.  Accordingly,  we  find  by  reference  to  scien- 
tific periodicals,  that  all  the  varied  phenomena  pre- 
sented by  the  comet  have  been  carefully  recorded, 
as  well  as  accurate  determinations,  from  night  to 
night,  of  its  position  in  the  heavens. 

The  comet  passed  the  perihelion  of  its  orbit,  or 
the  point  nearest  the  sun,  on  the  12th  of  September, 


GREAT   COMET   OF   1811.  109 

at  a  distance  of  98,565,000  miles.  On  the  7th  of 
September  the  tail  appeared  to  be  bent  off  in  two 
branches.  These  branches,  however,  did  not  pro- 
ceed from  the  head  of  the  comet,  but  seemed  to  be, 
as  it  were,  hung  together  at  a  slight  distance  from 
it,  and  separated  from  it  by  a  dark  interval.  The 
tail  was  then  five  degrees  in  length,  but  on  Septem- 
ber 20th  it  had  increased  to  10  degrees.  On  the  llth 
of  October  the  length  of  the  train  was  only  about 
13  degrees,  while  a  few  days  previous  it  was  found 
to  be  25  degrees  long,  and  6  degrees  broad  at  the 
extremity.  The  central  condensation  of  nebulous 
matter  was  50,000  miles  in  diameter,  or  more  than 
six  times  the  diameter  of  the  earth.  The  envelope 
was  30,000  miles  in  thickness,  and  the  centre  of  the 
nucleus  was  separated  from  its  interior  surface  by  a 
space  of  36,000  miles,  so  that  the  diameter  of  the 
head  of  the  comet  must  have  been  132,000  miles. 
In  the  centre  of  the  nucleus  there  was  a  brilliant 
point  428  miles  in  diameter,  which  was  probably 
composed  of  solid  matter.  The  part  of  the  nucleus 
immediately  surrounding  this  may  be  supposed  to 
have  been  fluid,  and  those  portions  still  more  remote 
to  have  been  composed  of  a  sort  of  gaseous  or  ele- 
mentary form  of  sidereal  matter.  It  was  imagined 
by  some  that  the  comet  had  a  sort  of  phosphorescent 
light  of  its  own,  proceeding  from  the  denser  portion 
of  the  nucleus.  The  coma  was  extremely  rarefied, 
10 


110  TREATISE   ON    COMETS. 

and  presented  the  appearance  of  a  very  faint  whitish 
light,  scattered  in  separate  portions.  At  the  time 
when  the  train  was  seen  divided  into  two  parts,  this 
apparent  division  of  the  coma  was  very  perceptible, 
and  the  part  which  directly  encompassed  the  nucleus 
was  much  brighter  than  the  other  portion,  which 
shone  with  a  faint  greyish  light,  and  sweeping  around 
the  former  at  a  distance,  formed  the  double  tail.  The 
phenomena  thus  presented  made  the  comet  appear 
somewhat  like  a  very  brilliant  object  surrounded  by 
a  dense  net-work  of  gauze.  The  diameter  of  this 
exterior  envelope  was  more  than  500,000  miles,  and 
its  appearance  was  such  as  to  induce  Schroeter  to 
suppose  that  it  very  clearly  indicated  the  existence 
of  a  repulsive  force  residing  in  or  around  the  nucleus, 
but  of  what  character  he  was  unable  to  determine. 
Toward  the  latter  part  of  November  and  in  the  first 
week  in  December,  great  changes  were  observed  to 
take  place  in  the  form  and  magnitude  of  the  head 
of  the  comet.  The  nebulous  matter,  which  had  for 
several  months  exhibited  the  appearance  of  extreme 
rarefaction,  was  seen  to  be  condensed  or  attracted 
towards  the  nucleus ;  and  the  changes  were  so  rapid 
that  there  was  exhibited  the  most  incontrovertible 
proof  of  physical  action  on  a  grand  scale,  perhaps 
such  that  no  phenomena  which  have  ever  been  wit- 
nessed on  our  earth,  will  serve  to  give  us  any  adequate 
conception  of  its  magnitude. 


GREAT   COMET   OF   1811.  Ill 

The  position  of  the  comet,  when  brightest,  with 
respect  to  the  earth  and  sun,  was  such  that  although 
the  train  at  one  time  extended  to  a  distance    of 
120,000,000  miles,  it  did  not  subtend  an  angle  of 
more  than  20  degrees.     Under  more  favorable  cir- 
cumstances as  regards  position,  it  might  have  been 
50  degrees  in  length.     In  the  case  of  this  comet, 
also,  streamers  were  seen  to  shoot  out  from  the  con- 
vex side  of  the  tail  to  a  distance  of  more  than  two 
millions  of  miles,  thus  producing  to  a  limited  extent 
the  appearance  of  coruscations  such  as  were  pre- 
sented by  the  comet  of  1807.     The  breadth  of  the 
tail,  at  its  extremity,  was  nearly  fifteen  millions  of 
miles.     The  comet  approached  the    earth    only  to 
within  about  110,000,000  miles.     It  was  supposed 
by  Herschel,  that  the  solid  portion  of  the  nucleus 
was  spherical,  that  it  shone  in  part  by  its  own  native 
light,  and  that  it  probably  had  a  rotation  around  an 
axis. 

The  elements  of  the  orbit  of  this  comet  have  been 
investigated  by  several  astronomers,  and  it  has  been 
found  to  be  elliptical,  although  the  periods  of  revo- 
lution assigned  by  different  computers  vary  between 
long  limits.  The  most  complete  research,  however, 
was  performed  by  Argelander,  who  found,  from  a 
discussion  of  the  entire  series  of  observations  worthy 
of  confidence,  that  the  period  of  revolution  of  the 
comet  is  3,065  years.  Bessel,  by  a  similar  process 


112  TREATISE   ON   COMETS. 

of  investigation,  and  taking  into  account  the  dis- 
turbances produced  by  the  attractions  of  the  planets 
—  which  will  subsequently  be  explained  —  finds  its 
period  to  be  3,383  years.  Its  aphelion  distance  is, 
therefore,  more  than  one  hundred  and  fifty  thousand 
millions  of  miles.  But  great  as  this  distance  is,  it 
is  perfectly  certain  that  there  are  many  comets  which 
revolve  in  orbits  far  more  extensive  than  the  one 
described  by  this  comet.  Indeed,  there  seems  to  be 
no  limit  to  the  distance  to  which  these  erratic  worlds 
may  sweep  outward  from  the  sun ;  and  their  return 
depends  simply  on  the  fact,  whether  they  recede  so 
far  as  to  fall  within  the  attractive  influence  of  some 
other  sun,  toward  which  they  begin  to  urge  their 
flight,  and  through  whose  system  of  planets  they 
carry  the  same  apprehensions  of  danger  which  they 
have  caused  in  our  own  system,  or  more  especially 
to  the  inhabitants  of  our  earth.  Thus  may  the 
comets  be  regarded  a  connecting  link  between  the 
systems  of  suns  and  planets  which  fill  the  immensity 
of  space. 

The  great  comet  of  1811  may  justly  be  considered 
as  the  most  magnificent  one,  in  all  respects,  which 
has  ever  visited  our  sun.  In  the  case  of  nearly  all 
the  very  bright  comets,  the  perihelion  distance  has 
been  small ;  but  in  the  case  here  presented  it  was 
considerably  more  than  the  mean  distance  of  the 
earth  from  the  sun.  The  other  bright  comets  have 


COMETS   FROM    1819   TO    1830.        113 

usually  approached  very  near  the  earth,  while  this 
one  was  even  more  remote  from  us  than  from  the 
sun.  Yet  under  all  these  circumstances  its  brilliancy 
has  rarely,  if  ever,  been  excelled,  while  in  magni- 
tude it  was  surpassed  only  by  the  sun  itself.  It  is 
no  wonder,  therefore,  that  to  those  unacquainted 
with  the  nature  of  such  bodies,  it  was  a  source  of 
great  alarm ;  or  that  even  among  the  more  intelli- 
gent population  of  Europe,  it  was  regarded  as  having 
produced  a  most  wonderful  effect  on  the  climate  of 
the  earth ;  although  astronomers  have  shown  that 
such  was  not  the  case. 

For  a  period  of  thirty  years  immediately  following 
the  disappearance  of  the  comet  of  1811,  the  dis- 
covery of  these  mysterious  bodies  was  of  frequent 
occurrence ;  but  only  in  a  few  instances  were  they 
of  sufficient  brilliancy  to  be  visible  to  the  naked  eye. 
Early  in  July,  1819,  there  was  one  discovered  in 
Europe  which  became  pretty  bright,  and  had  a  tail 
about  8  degrees  in  length.  In  the  years  1821,  1822, 
and  1823  there  were  comets  visible  to  the  naked  eye, 
one  in  each  year,  and  the  last  one  was  remarkable 
for  having  exhibited  the  phenomenon  of  having,  in 
addition  to  the  usual  tail,  another  one  in  a  contrary 
direction,  varying  from  four  to  seven  degrees  in 
length.  In  1825  and  1826  there  were  comets  of 
considerable  size  ;  and  in  1830  there  were  two  which 
were  distinctly  visible  to  the  naked  eye,  one  of 
10*  H 


114  TREATISE   ON    COMETS. 

which,  discovered  at  sea  on  March  17,  had  a  tail 
about  8  degrees  in  length,  with  a  very  bright  nucleus. 
In  1835  the  comet  of  Halley  made  its  appearance,  in 
accordance  with  the  prediction  ;  and,  in  the  southern 
hemisphere,  presented  a  very  fine  appearance.  On 
the  6th  of  March,  1840,  a  comet  was  discovered  by 
Galle,  at  Berlin,  which  had  a  tail  several  degrees  in 
length.  These  comprise  the  principal  comets,  with 
the  exception  of  two  or  three  periodic  ones  to  be 
described  hereafter — which  had  appeared  from  1811 
up  to  this  date,  and  do  not  afford  any  instance  of 
one  of  sufficient  interest  to  merit  a  particular  de- 
scription. In  the  case,  however,  of  the  comet  which 
appeared  in  March,  1843,  we  are  permitted  to  con- 
template phenomena  of  a  somewhat  different  cha- 
racter from  those  presented  by  the  great  comet  just 
described,  but  in  which  there  is  fully  as  much  of  the 
very  highest  interest. 

On  the  morning  of  the  28th  of  February,  1843, 
soon  after  sunrise,  a  brilliant  object  was  seen  only 
two  or  three  degrees  from  the  sun,  by  numerous 
observers  in  different  parts  of  the  globe.  It  was 
seen  at  the  Cape  of  Good  Hope  a  little  before  sun- 
set, and  at  several  places  in  the  New  England  States 
as  early  as  half  past  seven  o'clock  in  the  morning. 
In  the  early  part  of  the  afternoon  it  was  seen  by  the 
naked  eye  as  a  bright  light,  resembling  a  dagger  in 
form,  and  presented  at  once  the  appearance  of  a 


GREAT   COMET   OF   1843.  115 

very  brilliant  comet.  The  length  of  the  head  and 
tail  was  about  equal  to  twice  the  diameter  of  the 
sun,  or  about  one  degree.  The  head  is  described  at 
this  time  as  appearing  circular,  when  viewed  with 
the  naked  eye ;  its  light  equal  to  that  of  the  moon 
at  midnight,  in  a  clear  sky ;  and  its  apparent  size 
about  one-eighth  the  area  of  the  full  moon.  The 
train  shone  with  a  paler  light,  gradually  diverging 
from  the  nucleus,  and  fading  away  by  degrees  in 
the  intensely  bright  sky.  It  was  observed  at  Wood- 
stock, Vermont,  with  a  telescope  of  small  magnify- 
ing power,  and  is  described  as  presenting  a  distinct 
and  most  beautiful  appearance, —  exhibiting  a  very 
white  and  bright  nucleus,  and  a  tail  divided  near 
the  nucleus  into  two  separate  branches.  At  six 
minutes  after  three  o'clock,  on  the  afternoon  of  the 
same  day,  the  distance  of  the  nucleus  of  the  comet 
from  the  sun  was  measured  by  Captain  Clark,  at 
Portland,  Maine.  The  distance  of  the  nucleus  from 
the  sun's  farthest  limb  was  found  to  be  4  degrees, 
6  minutes,  and  15  seconds  of  arc ;  or  3  degrees,  50 
minutes,  and  43  seconds  from  the  centre.  The  head 
of  the  comet,  and  also  every  part  of  the  tail,  were 
then  described  as  being  as  well  defined  as  the  moon 
on  a  clear  day.  The  nucleus  and  tail  presented  the 
same  appearance,  and  resembled  a  perfectly  pure 
white  cloud,  without  any  variation,  except  a  slight 
change  near  the  head,  just  sufficient  to  distinguish 


116  TREATISE   ON    COMETS. 

the  nucleus  from  the  tail  at  that  point.  In  fact,  the 
nucleus  appeared  so  dense  that  it  was  probable  that 
it  might  have  been  visible  on  the  sun's  disc,  in  case 
it  had  passed  between  this  and  the  observer.  The 
entire  length  of  the  comet,  as  already  remarked, 
was  at  this  time  about  twice  the  diameter  of  the 
sun ;  and  since  the  distance  of  the  comet  and  sun 
from  the  earth  were  nearly  the  same,  as  will  soon 
be  evident,  we  shall  find  that  the  length  of  the 
nucleus  and  train  combined  was  1,700,000  miles. 

The  comet  was  observed  also  by  means  of  a  sex- 
tant, in  Mexico,  by  Mr.  Bowring,  from  nine  o'clock 
in  the  morning  of  the  28th  of  February,  until  near 
sunset,  and  its  altitude  repeatedly  measured.  It 
was  seen  on  the  same  day  at  Boulogne,  Parma,  and 
Genoa,  in  Italy ;  and  it  is  reported  also  that  it  was 
seen  on  the  27th  of  February,  by  Captain  liay,  at 
Conception,  in  South  America.  It  was  then  east  of 
the  sun,  and  distant  from  the  nearest  limb  about 
one-sixth  of  his  diameter,  or  about  130,000  miles. 
It  was  seen  at  Pernambuco,  in  Brazil,  and  in  Van 
Dieman's  Land,  on  the  1st  of  March ;  and  the  fol- 
lowing day  it  w^as  very  generally  observed  in  the 
tropical  latitudes  in  the  northern  hemisphere.  On 
the  3d,  the  head  was  so  far  disengaged  from  the 
rays  of  the  sun,  as  to  be  visible  above  the  horizon 
for  a  short  time  after  sunset ;  and  when  seen  through 
a  telescope  of  considerable  power,  it  presented  the 


GREAT   COMET   OF   1843.  117 

appearance  of  a  planetary  disc,  from  which  were 
divergent  rays  in  the  direction  of  the  tail.  The  tail 
is  said  to  have  appeared  double,  consisting  of  two 
principal  lateral  streamers,  making  a  very  small 
angle  with  each  other,  and  divided  by  a  compara- 
tively dark  line,  and  to  have  been  about  25  degrees 
in  length.  It  seemed,  however,  to  be  prolonged,  on 
the  north  side,  by  a  divergent  streamer,  making  an 
angle  of  at  least  5  degrees  with  the  general  direction 
of  the  axis,  and  plainly  distinguishable  as  far  as  65 
degrees  from  the  head.  A  similar,  but  fainter  pro- 
longation, is  said  to  have  been  visible  on  the  south 
side.  The  representations  of  the  comet  at  this  time, 
show  it  to  be  highly  symmetrical,  and  give  it  the 
appearance  of  a  vivid  cone  of  light,  with  a  dark 
axis,  and  nearly  rectilinear  sides,  inclosed  in  a  fainter 
cone,  the  sides  of  which  curve  slightly  outwards. 

On  the  4th  of  March,  the  comet  was  seen  at  New 
York ;  and  on  the  evening  of  the  5th,  it  was  gene- 
rally noticed  throughout  the  United  States.  From 
this  date  until  the  beginning  of  April,  it  presented 
a  most  magnificent  appearance  in  the  western  hea- 
vens, and  is  said  to  have  been  so  brilliant  in  some 
places  as  to  throw  a  strong  light  on  the  sea.  The 
nucleus,  however,  did  not  remain  brilliant  for  any 
considerable  portion  of  time.  On  the  3d  of  March 
it  was  as  bright  as  a  star  of  the  first  magnitude ;  on 
the  llth  it  was  not  brighter  than  a  star  of  the  third 


118  TREATISE   ON    COMETS. 

magnitude;  and  on  the  19th  and  20th  it  had  tecome 
so  faint  that  it  could  not  be  discerned  without  the 
aid  of  a  telescope.  The  tail  still  preserved  its  bril- 
liancy, and  remained  visible  as  a  great  beam  of 
nebulous  light,  until  the  first  week  in  April. 

The  first  regular  observation  of  the  comet,  with  a 
suitable  instrument,  was  made  at  the  Cape  of  Good 
Hope  on  the  3d  of  March,  after  which  it  was  regu- 
larly observed  until  its  final  disappearance — the  last 
recorded  observation  having  been  made  at  Berlin, 
on  the  15th  of  April.  It  was  not  seen  in  England 
till  the  17th  of  March,  and  then  only  as  an  immense 
stream  of  light,  without  any  appearance  of  a  nucleus. 
The  first  observation  of  the  nucleus  in  Europe  was 
made  at  Rome  and  Naples  on  the  17th  of  March, 
and  it  was  subsequently  observed  at  all  the  obser- 
vatories on  the  continent.  The  opposite  diagram 
represents  the  comet  as  it  appeared  to  the  naked 
eye  in  the  tropical  latitudes,  early  in  March. 

The  greatest  length  of  the  tail  was  on  the  5th  of 
March,  when  it  was  about  70  degrees,  and  was  ob- 
served to  be  slightly  curved.  Its  breadth  was  a 
little  more  than  a  degree.  In  the  tropical  latitudes 
the  tail  was  about  sixty  degrees  in  length,  and  the 
nucleus  shone  forth  with  almost  unexampled  lustre, 
creating  the  most  profound  sensations  of  astonish- 
ment and  admiration  wherever  it  was  seen.  On  the 
llth  of  March,  as  seen  at  Calcutta,  the  tail  was 


GREAT   COMET   or   1843.  119 

observed  to  have  shot  forth  a  lateral  ray  or  train, 
nearly  twice  as  long  as  the  regular  one,  but  fainter, 
and  making  an  angle  of  about  18  degrees  with  its 
direction,  on  the  southern  side.  This  projection  of 
a  secondary  train  to  such  an  enormous  length  —  it 
having  been  nearly  100  degrees — was  not  observed 
either  before  or  after  this  date.  The  length  of  the 
tail  did  not  vary  much  after  it  had  attained  its  max- 
imum, but  remained  nearly  stationary,  in  this  re- 
spect, until  the  first  week  in  April,  when  it  ceased 
to  be  visible  to  the  naked  eye,  owing  to  the  increased 
distance  of  the  comet  from  the  earth.  The  appa- 
rent length  and  size  of  the  train,  although  enor- 
mous, hardly  serve  to  give  a  definite  idea  of  its 
absolute  size.  The  greatest  apparent  length  was  at 
the  time  when  the  comet  was  nearest  the  earth,  but 
when  it  was  still  distant  more  than  80,000,000  miles. 
Let  us  suppose,  therefore,  that  the  length  of  the 
tail  was  found  to  be  60  degrees,  or  about  115  times 
the  apparent  diameter  of  the  sun,  and  we  shall  find 
its  absolute  length  to  be  86,000,000  miles.  At  the 
time  of  the  discovery  of  the  .comet,  in  the  daytime, 
the  tail  was  turned  nearly  toward  the  earth,  and 
consequently  appeared  brighter,  yet  shorter,  than 
would  otherwise  have  been  the  case.  Had  it  been 
at  right  angles  to  the  plane  of  the  orbit  of  the  earth, 
supposing  its  apparent  length  to  have  been  the  same 
as  actually  observed,  its  actual  length  would  not 


120 


TREATISE   ON   COMETS. 


have  exceeded  2,000,000  miles.  It  is  not  certainly 
known  whether  the  comet  had  a  train  of  any  con- 
siderable extent  previous  to  its  perihelion  passage. 
It  is  probable,  however,  that  it  had ;  but  from  what 
is  now  known  of  the  development  of  the  tails  of 
comets,  it  could  not  have  been  near  as  large  as  im- 
mediately after  the  perihelion  passage.  The  length 
of  the  tail  for  each  day  during  which  the  comet  was 
visible,  will  serve  to  show  the  progress  of  its  devel- 
opment, and  is  exhibited  in  the  following  table : 


Date. 

Length  of  tail 
in  miles. 

Date. 

Length  of  tail 
in  miles. 

Feb'y  28,    . 

.     .     35,000,000 

March  15,   ..,,» 

.  105,000,000 

March    1,    . 

.     .     55,000,000 

"      17,   .    . 

.  106,000,000 

"        2,   . 

.     .     70,000,000 

"      19,    .     . 

.  107,000,000 

3,   . 

.     .     82,000,000 

"      21,    .     . 

.  108,000,000 

4,   . 

.     .    91,000,000 

"      23,   .     . 

.  109,000,000 

"        5,    . 

.     .     96,000,000 

"      25,   .     . 

.  108,000,000 

6,    . 

.     .     99,000,000 

"      27,   .     . 

.  107,000,000 

7,   . 

.     .  100,000,000 

"      29,   .    . 

.  105,000,000 

8,   . 

.     .  101,000,000 

April      1,   .     . 

.  102,000,000 

9,   . 

.     .  102,000,000 

3,    .     . 

.     99,000,000 

"      10,    . 

.     .  102,800,000 

5,    .     . 

.     95,000,000 

"      11,    . 

.     .  103,000,000 

"        7,   .    . 

.     89,000,000 

"       13,    . 

.     .  104,000,000 

The  breadth  of  the  tail  at  the  extremity  varied 
from  one  to  three  millions  of  miles.  The  length  of 
the  streamer  seen  at  Calcutta  on  the  llth  of  March, 
must  have  been  about  150,000,000  miles ;  and  if  we 
suppose  this  to  have  been  projected  in  a  single  day, 
or  perhaps  in  much  less  than  24  hours,  it  seems 


GREAT   COMET   OF   1843.  121 

almost,  if  not  actually  impossible,  to  form  any  con- 
ception of  the  forces  which  must  have  operated  to 
produce  such  a  velocity  of  projection  of  material 
substance  through  space.  It  is  certain,  at  least,  that 
the  velocity  must  have  been  such  as  no  other  natural 
phenomenon  is  capable  of  exciting;  and  that  in  cases 
similar  to  this  we  have  to  deal  with  forces  incom- 
parably superior  in  energy  to  any  with  which  we 
are  acquainted,  and  with  matter  whose  inertia  must 
be  extremely  insensible. 

The  head  of  the  comet,  when  compared  with  the 
dimensions  of  the  tail,  was  small.  It  was  hardly 
perceptible  even  on  the  28th  of  February,  when  its 
brilliancy  was  greatest  —  the  comet  presenting  the 
appearance  of  a  mere  point,  with  a  divergent  mass 
of  matter  in  the  form  of  a  tail.  For  this  reason  its 
cometary  nature  was  at  first  doubted  by  those  unac- 
quainted with  such  objects.  A  few  days  later  the 
head,  although  faint  in  comparison  with  the  tail, 
could  be  very  distinctly  seen,  but  remained  visible 
to  the  naked  eye  only  for  a  very  few  days.  During 
the  last  days  on  which  the  comet  was  observed  with 
large  telescopes,  it  was  almost  impossible  to  distin- 
guish the  nucleus,  even  while  the  train  was  yet 
brilliant  to  the  unassisted  eye.  It  was  for  this  rea- 
son that  Bessel,  in  speaking  of  the  appearance  of 
the  comet,  remarked  that  it  seemed  to  have  ex- 
hausted its  head  in  the  manufacture  of  its  tail  —  a 
11 


122  TREATISE   ON   COMETS. 

remark  which,  when  we  come  to  consider  the  theory 
of  the  formation  of  the  tails  of  comets,  will  have 
due  significance.  The  diameter  of  the  head  of  the 
comet  was  probably  about  8000  miles,  and  the  abso- 
lute diameter  of  the  nebulosity  surrounding  the 
head  was  about  30,000  miles.  These  results,  in 
connection  with  the  dimensions  of  the  tail,  already 
given,  will  serve  to  exhibit  the  enormous  magnitude 
of  this  celestial  visitor. 

This  comet  did  not  exhibit  any  indication  of  a 
solid  nucleus,  but,  on  the  contrary,  considering  the 
size  and  brilliancy  of  the  head  a3  compared  with 
the  tail,  it  seems  certain  that  the  entire  comet  was 
composed  of  nebulous  matter  in  a  state  of  extreme 
tenuity  —  a  supposition  which  the  elements  of  its 
orbit  will  furnish  the  most  unequivocal  testimony 
to  confirm.  However  great,  then,  the  volume  of  the 
comet  may  have  been,  the  absolute  quantity  of 
matter  was  inconsiderable,  when  compared  with  even 
the  smallest  planets  of  our  system.  Indeed,  it  is 
highly  probable  that  the  very  smallest  members  of 
the  group  of  small  planets  between  Mars  and  Jupi- 
ter, those  whose  diameters  do  not  exceed  a  few 
hundred  miles,  greatly  exceed  in  weight  the  entire 
mass  of  the  comet.  The  question  may  therefore 
arise,  as  to  what  is  due  the  intensity  of  its  light 
when  seen  at  noonday,  on  the  28th  of  February,  in 
the  full  blaze  of  the  sun.  For  this  the  elements  of 


GREAT   COMET  OF   1843.  123 

the  orbit  furnish  a  ready  and  complete  method  of 
solution. 

The  elements  of  the  orbit  of  the  great  comet  of 
1843  may  well  be  said  to  be  among  the  most  re- 
markable of  those  which  have  hitherto  been  re- 
corded.    The  first  elements  which  were  computed 
actually  gave  the  perihelion  distance  of  the  comet, 
measured  from  the  sun's  centre,  as  less  than  the 
semi-diameter  of  the  sun,  which  is  a  physical  im- 
possibility, since  the  comet  was  observed  after  its 
perihelion  passage.     A  complete  discussion  of  all 
the  observations  taken  during  a  period  of  forty  days, 
made  the   nearest  approach  of  the  centre  of  the 
nucleus  to  the  centre  of  the  sun  only  390,000  miles, 
or  about  50,000  miles  below  his  surface.     Another 
determination    made    it   405,000    miles,    or   about 
36,000  miles  less  than  the  radius  of  the  sun.    Were 
these  results  correct,  the  comet  would  have  plunged 
directly  into  the  sun  in  passing  the  perihelion  of  its 
orbit,  and  would,  in  all  probability,  have  become 
absorbed  in  the  solar  atmosphere.     It  would,  there- 
fore, have  been  visible  only  before  the  perihelion 
passage,  which  took  place  on  the  evening  of  the 
27th  of  February,  the  day  previous  to  its  discovery7. 
The  average  distance  of  the  comet  in  its  perihelion, 
obtained  by  the  astronomers  who  were  engaged  in 
*  computations  respecting  its  orbit,  was  about  510,000 
miles.     If  we  adopt  this  distance, — and  there  is  the 


124  TREATISE   ON    COMETS. 

greatest  probability  of  its  correctness, — we  conclude- 
that  the  comet  approached  the  sun's  surface  within 
about  seventy  thousand  miles,  or  within  one-sixth 
of  his  radius.  "We  perceive,  therefore,  that  the 
comet  almost  literally  grazed  the  sun's  disc  —  a  fact 
which  serves  to  explain  the  great  brilliancy  of  the 
nucleus  at  this  time. 

It  is  well  known  that  the  intensity  of  both  the 
light  and  radiant  heat  of  the  sun,  at  different  dis- 
tances from  that  luminary,  increase  proportionally 
to  the  spherical  area  of  the  portion  of  the  visible 
hemisphere  covered  by  the  sun's  disc.  In  the  case 
of  the  earth,  this  disc  -has  an  average  diameter  of 
a  little  more  than  half  a  degree ;  but,  could  an  ob- 
server have  been  stationed  on  the  comet,  when  in 
the  perihelion,  the  apparent  angular  diameter  of  the 
sun.  would  have  been  nearly  125  degrees.  We  shall 
therefore  find,  from  the  principle  just  stated  in  refer- 
ence to  the  distribution  of  light  and  heat  by  the 
sun,  that  the  amount  of  light  and  heat  received  on 
equal  areas  of  exposed  surface  of  the  earth  and 
comet,  would  be  in  the  ratio  of  one  to  forty-nine 
thousand.  Now,  it  has  been  determined  by  experi- 
ment, that,  by  collecting  the  rays  of  the  sun  to  such 
an  extent  that  the  intensity  of  his  heat  and  light  as 
received  at  the  earth  is  increased  one  thousand 
times,  the  heat  produced  is  sufficient  to  melt  any 
mineral  substance  known  on  our  earth.  With  this 


GKEAT  COMET   OF   1843.  125 

heat  iron  is  melted  in  a  few  seconds,  and  such  min- 
erals as  cornelian,  agate,  and  rock  crystal.  The 
heat  of  the  sun,  as  received  at  the  comet,  must  have 
been  at  least  forty-nine  times  more  intense  than  the 
heat  thus  generated.  The  great  comet  of  1680  was 
distant  from  the  sun's  surface  about  150,000  miles, 
when  in  its  perihelion,  or  more  than  double  the  cor- 
responding distance  of  this  comet;  and  yet  it  was 
computed  by  Newton  that  the  former  was  subjected 
to  an  intensity  of  heat  2000  times  that  of  red-hot 
iron.  How  much  more  intense,  then,  must  have 
been  the  heat  received  by  the  comet  of  1843 !  Such 
a  temperature  would  actually  have  converted  our 
earth  into  vapor ;  or,  if  any  substance  retained  a 
solid  form,  it  would  have  been  in  a  state  of  the  most 
intense  ignition.  On  the  morning  of  the  28th  of 
February,  the  comet  must  have  been  literally  red 
hot,  although  evidently  in  a  gaseous  state ;  and,  from 
its  appearance  as  observed,  it  retained  its  heat  for 
several  days  after  its  perihelion  passage,  having 
presented  a  peculiar  fiery  appearance.  In  the  tropi- 
cal latitudes,  and  more  especially  in  the  vicinity  of 
the  equator,  it  was  described  as  resembling  a  stream 
of  fire  from  a  furnace.  It  may  be  conjectured, 
therefore,  that  in  case  the  comet  had  originally  a 
solid  nucleus,  it  was  converted  into  vapor  at  this 
approach  to  the  sun;  while  the  intensity  of  the  heat, 
and  the  effect  of  its  operation,  were  such  that  the 
11* 


126  TREATISE   ON   COMETS. 

comet  retained  this  gaseous  form  during  the  entire 
period  of  its  visibility  in  1843. 

The  comet  did  not,  however,  remain  long  in  a 
position  exposed  to  such  an  enormous  range  of  tem- 
perature. Its  velocity  at  this  time  was  not  less  than 
370  miles  in  a  second  of  time,  or  1,332,000  miles" 
per  hour,  which  would  be  sufficient,  if  undiminished, 
to  cause  the  comet  to  make  an  entire  revolution 
around  the  sun  in  two  hours  and  one-third.  This 
velocity  was  so  great  only  for  a  small  portion  of 
time,  and  rapidly  diminished  as  the  comet  proceeded 
in  its  orbit.  It  was  still  such  that  during  the  twelve 
hours  immediately  preceding  the  perihelion  passage, 
and  the  twelve  hours  immediately  following  it,  the 
comet  passed  over  an  arc  of  its  orbit  amounting  to 
290  degrees,  or,  in  other  words,  performed  more 
than  three-quarters  of  its  circuit  around  the  sun. 
During  the  brief  period  in  which  it  was  visible,  it 
had  described  173  degrees  of  its  orbit,  measured 
from  the  perihelion;  while  to  describe  the  next 
7  degrees  will  require  many  years,  at  least,  and  per- 
haps many  centuries.  It  thus  appears  that  in  about 
two  hours  the  .heat  of  the  sun,  as  experienced  by 
the  comet,  would  have  been  reduced  to  about  one- 
fourth  its  maximum  amount,  and  that  afterwards 
the  diminution  was  proportionally  greater.  In  this 
manner,  then,  we  may  explain  the  various  changes 
which  were  observed  to  take  place  in  the  brilliancy 


GREAT   COMET   OF   1843.  127 

of  the  comet,  and  also  the  extreme  intensity  of  its 
light  at  the  time  of  discovery. 

In  computing  the  elements  of  the  orbit  of  this 
comet,  it  was  found  by  many  astronomers  to  give 
strong  indications  of  ellipticity,  while  others  were 
led  to  conclude  that  the  entire  series  of  observations 
could  be  represented  within  the  limits  of  their  pro- 
bable errors,  by  supposing  the  orbit  to  be  a  parabola. 
Encke  obtained  a  hyperbolic  orbit,  whose  eccen- 
tricity, however,  differed  but  little  from  that  of  the 
parabola.  But  it  should  be  remarked  that  before 
any  calculations  had  been  made,  it  was  asserted 
that,  in  the  year  1668,  the  tail  of  an  immense  comet 
had  been  seen  at  Lisbon,  at  Boulogne,  at  several 
points  in  Brazil,  and  elsewhere,  occupying  nearly 
the  same  position  in  the  heavens,  and  at  the  same 
season  of  the  year.  Cassini  observed  the  tail  at 
Boulogne,  on  the  10th  of  March,  1668 ;  and  from 
this  determined  that  the  head  was  in  the  immediate 
vicinity  of  the  sun.  The  tail  was  about  50  degrees 
in  length,  and  was  so  brilliant  that  its  reflected  trace 
was  easily  distinguished  on  the  sea.  This  brilliancy, 
however,  lasted  only  for  a  few  days ;  and  on  one  or 
two  occasions  the  head  could  be  barely  distinguished 
in  the  twilight,  but  as  a  very  dim  object.  The  comet 
of  1668  was  not  very  accurately  observed,  and  its 
orbit  was  unknown ;  yet  the  strange  coincidence  of 
similar  situation,  season  of  the  year,  and  physical 


128  TREATISE   ON    COMETS. 

appearance,  excited  a  very  strong  suspicion  of  iden- 
tity, thus  assigning  to  the  comet  a  period  of  exactly 
175  years.  By  estimating  the  position  of  the  head 
from  the  observed  direction  of  the  tail,  Henderson 
was  enabled  to  determine  an  approximate  orbit  for 
the  comet  of  1668 ;  and  he  found,  strangely  enough, 
that  the  elements  closely  resembled  those  of  the 
comet  of  1843.  Assuming,  then,  their  identity,  and 
using  the  corrected  elements  of  the  latter  comet,  it 
was  found  that  the  accordance  was  still  better;  and 
it  was,  therefore,  announced  that  these  comets  not 
only  exhibited  the  same  appearance,  but  pursued 
very  nearly  the  same  path,  both  real  and  apparent. 
Again,  it  has  been  remarked  that  comets  are  re- 
corded to  have  been  seen  in  the  years  268,  442,  791, 
968,  1143,  1317,  and  1494,  which  may  have  been 
previous  returns  of  the  comet  now  under  considera- 
tion, since  a  period  of  175  years  would, — disregard- 
ing the  planetary  perturbations,  which,  from  the 
position  of  the  orbit,  must  necessarily  be  small, — 
make  it  appear  in  the  years  268,  443,  618,  793,  968, 
1143,  1318,  and  1493. 

Such  are  the  reasons  for  supposing  the  great 
comet  of  1843  to  have  a  periodic  time  of  175  years ; 
but  it  should  be  noticed  that  a  similar  comet  was 
also  seen  in  1689.  It  was  not  observed  in  Europe, 
but  was  seen  at  Pekin  and  in  the  southern  hemi- 
sphere, where  the  tail  was  estimated  at  upwards  of 


GH  EAT   COMET   OF   1843.  129 

60  degrees  in  length.  It  was  observed  from  the 
llth  to  the  23d  of  December ;  and  from  the  observa- 
tions, rude  as  they  are,  its  orbit  has  been  computed. 
Its  perihelion  distance  was  small,  and  the  places 
may  be  closely  represented  by  elements  similar  to 
those  of  the  comet  of  1843.  Those  who  have  felt 
inclined  to  consider  these  comets  identical,  have 
assigned  a  period  of  21  years  and  10  months,  which 
gives  seven  revolutions  between  1689  and  1843.  By 
this  means  the  identity  of  the  comets  of  1668, 1689, 
and  1843,  is  supposed  to  be  clearly  established ;  and 
the  fact  that  the  comet  has  not  been  observed  at  in- 
termediate returns,  is  accounted  for  by  the  position 
of  its  orbit,  which  is  such  that  the  comet  is  best  seen 
in  the  southern  hemisphere,  and  may  pass  unob- 
served in  the  northern  hemisphere.  If  we  adopt 
this  hypothesis,  the  comet  may  be  expected  to  reap- 
pear about  the  end  of  1864  or  beginning  of  1865, 
but  will  be  seen  only  in  the  southern  hemisphere. 

Some  astronomers  have  gone  still  further,  and 
have  supposed  the  comet  to  have  a  period  of  only  7 
years,  assigning  the  unfavorable  situation  of  its  orbit 
as  a  reason  for  its  not  having  been  seen  at  its  suc- 
cessive returns.  This,  however,  seems  wholly  in- 
admissible, since  a  comparison  of  all  the  best  obser- 
vations militates  against  the  hypothesis  of  an  ellipse 
of  short  period.  They  may  all  be  represented  by  a 
parabola  within  the  usual  limits  of  the  errors  of 

I 


130  TREATISE   ON   COMETS. 

cometary  observations,  although  an  ellipse  of  about 
180  years  represents  them  a  little  better.  Still  later 
determinations  of  the  orbit,  in  which  the  probable 
errors  of  the  observations  are  investigated,  and  the 
planetary  disturbances  duly  introduced,  increase  the 
period ;  and  in  one  instance  it  is  found  to  be  an 
ellipse  corresponding  to  a  period  of  revolution  of 
over  500  years.  It  is,  therefore,  extremely  doubtful 
whether  the  comets  of  1668,  1689,  and  1843,  are 
identical,  and  the  doubt  can  be  removed  only  by 
another  appearance  of  the  comet.  The  greatest 
probability  is  in  favor  of  the  identity  of  the  comets 
of  1668  and  1843  alone,  thus  establishing  a  period 
of  175  years  within  a  day  or  two,  more  or  less ;  yet 
in  this  case  there  exists  great  uncertainty,  and  such 
that  we  can  hardly  venture  to  predict  its  reappear- 
ance in  the  year  2018. 

The  most  then  which  we  can  say  respecting  the 
great  comet  of  1843,  is  that  the  prodigious  length 
of  its  tail  and  its  small  perihelion  distance,  both 
being  such  as  have  never  before  been  observed, 
invest  it  with  peculiar  interest ;  and  many  years  may- 
elapse  before  another  shall  appear  which,  in  so  short 
a  space  of  time,  will  excite  an  interest  so  universal, 
or  a  sensation  so  profound.  "We  have  been  minute 
in  the  description  of  the  details  of  its  appearance, 
in  order  that  the  degree  and  kind  of  interest  which 
is  attached  to  the  comets,  even  by  astronomers,  in 


COMETS   FROM   1845  TO    1853.        131 

the  present  state  of  the  science,  may  be  understood, 
and  also  for  the  purpose  of  exhibiting  the  important 
results  which  the  exact  calculations  of  modern  as- 
tronomy afford  us. 

The  next  comet  which  became  visible  to  the  naked 
eye,  was  discovered  by  Colla,  at  Parma,  on  the  2d 
of  June,  1845.  It  had  a  very  beautiful  tail,  about 
three  degrees  in  length.  It  has  been  supposed  to 
have  a  period  of  about  249  years,  making  it  identical 
with  a  comet  which  appeared  in  1596.  Another 
bright  comet  was  discovered  by  Hind,  at  London, 
on  the  6th  of  February,  1847.  It  passed  its  peri- 
helion on  the  30th  of  March,  at  a  distance  from  the 
sun  of  about  four  millions  of  miles,  and  was  so 
bright  that  it  was  observed  at  London  at  mid-day, 
when  only  a  few  degrees  from  the  sun.  A  comet 
was  also  discovered  by  Miss  Mitchell,  at  Kantucket, 
Mass.,  on  the  1st  of  October,  1847,  in  the  vicinity 
of  the  north  pole  of  the  heavens,  which  subsequently 
increased  in  brilliancy,  and  on  October  6th  became 
visible  to  the  naked  eye.  It  continued  to  increase 
in  brilliancy  for  several  days,  when  it  was  obscured 
by  bright  moonlight.  The  tail  was  very  faint,  and 
did  not  exceed  two  degrees  in  length.  It  passed  the 
perihelion  of  its  orbit  on  the  14th  of  November. 

On  the  10th  of  June,  1853,  a  faint  telescopic  object, 
which  was  subsequently  found  to  be  a  comet,  was 
discovered  by  Klinkerfues,  at  Gottingen.  It  con- 


132  TREATISE   ON   COMETS. 

tinned  to  increase  in  brilliancy,  and  on  the  7th  of 
August  began  to  be  faintly  visible  to  the  naked  eye. 
On  the  20th  of  August  it  was  as  bright  as  stars  of 
the  third  magnitude,  and  ten  days  later  it  equalled 
in  brilliancy  the  brightest  stars  of  the  first  magni- 
tude. On  the  31st  of  August  it  was  observed  at 
Olmiitz  in  the  day  time,  when  only  twelve  degrees 
from  the  sun ;  and  on  the  2d,  3d,  and  4th  days  of 
September  it  was  observed  at  noon,  although  only 
seven  degrees  from  the  sun.  It  was  also  seen  at 
Liverpool  on  the  3d  of  September,  about  noon,  by 
means  of  a  telescope.  The  first  appearance  of  a  tail 
was  about  the  beginning  of  August,  when  it  had 
attained  a  length  of  nearly  a  quarter  of  a  degree. 
On  the  22d  of  August  it  was  nearly  two  degrees  in 
length,  and  for  a  little  more  than  a  week  continued 
to  increase,  until  it  attained  a  maximum  length  of 
fifteen  degrees.  During  this  time  it  presented  a 
very  beautiful  appearance  in  the  western  heavens, 
soon  after  sunset,  to  the  naked  eye,  and  its  motions 
were  watched  with  no  small  degree  of  interest.  It 
passed  its  perihelion,  or  point  of  its  orbit  nearest 
the  sun,  on  the  1st  of  September;  but  it  is  not  known 
to  have  been  observed  in  the  northern  hemisphere 
after  the  4th  of  September.  It  was  seen  at  the  Cape 
of  Good  Hope  on  September  llth,  and  was  observed 
until  the  llth  of  January,  1854.  It  was  also  ob- 
served at  Santiago,  in  Chili,  from  the  16th  of  Sep- 


ORE  AT   Co  MET   OF   1858.  183 

tember  till  the  7th  of  October,  and  at  Kew  Zealand 
from  September  14th  to  October  10th.  About  the 
middle  of  September  the  tail  was  five  degrees  in 
length,  from  which  time  its  brightness  rapidly 
diminished,  till  it  became  invisible  to  the  naked 
eye.  The  orbit  is  found  to  be  a  parabola,  and,  con- 
sequently, the  comet  will  never  reappear  to  the  in- 
habitants of  our  earth,  but  will  pass  on  in  its  cease- 
less wanderings  to  visit  other  suns  and  systems. 

On  the  evening  of  the  2d  of  June,  1858,  a  faint 
nebulosity  was  discovered  by  Donati,  an  Italian 
astronomer,  at  Florence,  in  the  constellation  Leo, 
which  the  observations  of  a  few  days  proved  to  be  a 
comet.  It  was  then  very  far  distant  from  both  the  earth 
and  sun,  and  had  also  a  very  slow  motion  as  seen 
from  the  earth.  The  comet  was  seen  at  nearly  all 
the  observatories  in  Europe  in  less  than  a  week  after 
the  announcement  of  its  discovery.  The  news  of 
the  discovery  did  not  reach  America  until  about  the 
1st  of  July,  since  it  was  not  generally  announced  in 
Europe  until  near  the  middle  of  June.  The  comet, 
however,  had  already  been  independently  discovered 
by  Tuttle,  at  Cambridge,  Massachusetts,  on  the 
28th  of  June ;  by  Parkhurst,  at  Perth  Amboy,  $"ew 
Jersey,  on  the  29th  of  June,  and  by  Miss  Mitchell, 
at  JSTantueket,  on  the  1st  of  July,  each  observer 
being  at  the  time  unaware  of  its  previous  discovery 
in  Italy.  It  was  immediately  observed  at  all  the 
12 


134  TREATISE   ON    COMETS. 

principal  observatories  in  the  United  States,  and 
henceforward  observations  became  general  on  both 
continents.  As  soon  as  a  sufficient  number  of 
observations  had  been  taken,  the  elements  of  its 
orbit  were  computed,  and  it  was  found  that  the 
comet  was  distant  from  the  earth,  at  the  time  of 
its  discovery  by  Donati,  about  240,000,000  miles, 
and  that  it  was  slowly  approaching  the  sun,  and 
receding  from  the  earth.  The  distance  of  the 
comet  from  the  earth  attained  its  maximum  about 
the  middle  of  June,  when  it  remained  stationary  for 
a  few  days  in  respect  to  its  geocentric  distance.  It 
soon  began  to  approach  the  earth,  and  it  was  found 
that  its  brilliancy  would  be  subsequently  increased 
nearly  three  hundred  times.  On  account  of  the  slow 
motion  of  the  comet,  and  its  great  distance,  it  was 
found  to  be  difficult  to  determine  the  time  of  its 
perihelion  passage  with  any  considerable  precision. 
Indeed,  the  computations  of  different  astronomers 
exhibited  a  great  difference  in  this  element ;  and  it 
was  not  till  near  the  middle  of  August  that  its  future 
course  could  be  satisfactorily  ascertained.  It  was 
then  found  that  the  comet  would  pass  the  point  of 
its  orbit  nearest  the  sun  on  the  29th  of  September, 
that  it  would  become  visible  to  the  naked  eye  early 
in  September,  and  that  during  the  latter  part  of  this 
month,  and  the  first  half  of  October,  it  would 
present  a  magnificent  appearance  in  the  western 


GREAT   COMET   OF   1858.  135 

heavens,  soon  after  sunset.  It  was  found  also  that 
the  comet  would  be  so  situated  with  respect  to  the 
earth  and  sun,  that  it  would  not  only  be  visible  in 
the  evening  after  sunset,  but  during  the  greater  part 
of  September  it  would  rise  several  hours  before  the 
sun,  and,  consequently,  would  be  favorably  situated 
for  observation  in  the  morning. 

The  predictions  were  fully  realized.  The  comet 
was  distinctly  visible  to  the  naked  eye  on  September 
10th,  and  continued  to  increase  rapidly  in  brilliancy 
until  about  the  5th  of  October,  when  it  had  attained 
its  maximum  in  this  respect.  It  ceased  to  be  visible 
in  northern  latitudes,  except  near  the  equator,  about 
the  20th  of  October,  but  was  observed  in  the  south- 
ern hemisphere  from  the  beginning  of  October  to 
the  middle  of  May,  1859.  The  first  appearance  of 
a  tail  was  noticed  about  the  20th  of  August,  with 
the  aid  of  a  telescope,  and  early  in  September  it 
was  very  distinctly  noticed  to  be  curved.  Between 
the  10th  and  25th  of  September,  the  comet  increased 
rapidly  in  brilliancy,  and  the  train  was  lengthened 
proportionally.  The  comet  was  now  rapidly  ap- 
proaching both  the  earth  and  sun,  and  under 
circumstances,  as  we  shall  subsequently  notice,  pe- 
culiarly favorable  to  the  development  of  the  tail. 
Another  fortunate  circumstance  to  be  noticed  in 
this  connection,  was  that  the  moonlight,  about  the 
time  of  the  greatest  brilliancy  of  the  comet,  ceased 


136  TREATISE   ON   COMETS. 

to  interfere  with  the  observation  of  the  minutest 
details  of  its  form,  and  contributed  largely  to  en- 
hance its  general  appearance.  The  return  of  the 
moonlight  just  before  it  ceased  to  be  visible  above 
the  horizon  in  the  south-west,  and  when  its  brilliancy 
was  speedily  on  the  decline,  caused  its  decrease  and 
final  disappearance  to  be  fully  as  sudden  as  had 
been  its  increase  and  maximum  intensity  of  light. 

At  the  time  of  its  perihelion  passage  the  comet 
was  distant  from  the  sun  about  55,000,000  miles. 
Its  nearest  approach  to  the  earth  took  place  on  the 
llth  of  October,  at  which  date  it  was  distant  from 
us  about  50,000,000  miles.  The  following  table 
gives  the  distance  of  the  comet  from  the  earth  and 
sun,  and  also  its  hourly  motion  in  its  orbit,  for  differ- 
ent dates  during  the  period  of  its  visibility  in  the 
northern  hemisphere. 

Distance  from  gun    Distance  from  earth      Hourly  velocity 


UU.IV. 

in  miles. 

in  miles. 

in  miles. 

June  2,  . 

.  .  214,000,000 

240,000,000 

65,000 

July  1,  . 

.  .  175,000,000 

241,000,000 

72,000 

Aug.  1,  . 

.  .  128,000,000 

221,000,000 

84,000 

Sept.  1,  . 

.  .   82,000,000 

160,000,000 

105,000 

"  15,  . 

.  .   64,000,000 

116,000,000 

119,000 

Oct.  1,  . 

.  .   56,000,000 

66,000,000 

128,000 

"   11,  • 

.  .   61,000,000 

52,000,000 

123,000 

"   21,  . 

.  .   71,000,000 

67,000,000 

114,000 

The  brilliancy  of  a  comet,  as  seen  from  the  earth, 
varies  inversely  as  the  product  of  the  squares  of  its 
distance  from  the  earth  and  sun  respectively ;  and 


GREAT   COMET   OF   1858.  137 

it  results,  therefore,  from  the  foregoing  table,  that  this 
comet  was  200  times  brighter  on  the  1st  of  October 
than  at  the  time  of  its  discovery.  The  brightest 
stars  in  the  heavens  are  about  100  times  brighter 
than  those  which  can  be  barely  distinguished  by  the 
naked  eye.  This  fact  may  serve  to  give  an  idea  of 
the  comparative  brilliancy  of  the  comet  at  these  two 
dates. 

The  tail  of  this  comet  is  certainly  the  most  re- 
markable feature  to  be  considered.  At  the  time 
when  the  comet  was  nearest  the  earth,  it  could  be 
distinctly  traced  to  a  distance  from  the  nucleus  of 
nearly  70  degrees,  and  was  curved  in  the  form  of  a 
sabre.  On  the  20th  of  September,  it  was  first  ob- 
served to  be  bifurcated,  or  divided  into  two  branches 
or  streams.  The  dark  space  between  these  two 
streams  of  nebulous  light  was  directly  behind  the 
nucleus,  and  gave  it  the  appearance  of  the  shadow 
of  the  latter  obscuring  the  light  of  the  tail.  The 
southern  portion  of  the  tail  was  much  more  brilliant 
than  the  northern  portion ;  and  the  difference  was 
such  that,  in  the  twilight,  the  latter  could  not  be 
distinguished,  so  that  the  tail  appeared  single.  The 
length  of  the  tail,  at  this  time,  was  about  5  degrees. 
On  the  24th  of  September  the  length  of  the  tail  was 
more  than  7  degrees,  and  the  curvature  began  to  be 
very  plainly  exhibited.  On  the  27th  its  length  was 
13  degrees,  and  presented  the  phenomenon  of  a 
12* 


188  TREATISE    ON    COMETS. 

streamer,  or  supplementary  tail,  issuing  from  the 
convex  side  of  the  principal  one,  and  nearly  in  the 
direction  of  a  straight  line  drawn  from  the  sun 
through  the  nucleus.  From  this  date  until  the  10th 
of  October,  the  tail  increased  rapidly  in  brilliancy 
and  length  ;  and  from  the  phenomena  which  it  pre- 
sented, there  is  every  reason  to  believe  that  it  was 
directly  connected  with  the  changes  which,  as  we 
shall  notice,  were  continually  taking  place  in  the 
matter  composing  the  envelope  and  nucleus.  At 
times,  rays  or  jets  of  light  were  seen  streaming  in 
different  directions  from  the  centre ;  and  corusca- 
tions were  seen,  precisely  as  in  the  case  of  several 
comets  which  have  already  been  described.  On  the 
5th  of  October  there  were  two  streamers  distinctly 
visible,  one  of  which  was  more  than  fifty  degrees  in 
length,  corresponding  to  upwards  of  50,000,000 
miles.  At  this  time,  the  comet  was  so  bright  that 
not  only  the  nucleus,  but  also  a  portion  of  the  en- 
velope and  tail  could  be  seen,  by  means  of  a  tele- 
scope, before  sunset.  It  was  visible  to  the  naked 
eye  in  the  bright  twilight ;  and,  when  viewed  with 
a  telescope,  a  secondary  envelope  could  be  distinctly 
noticed.  At  the  point  where  the  curvature  of  the 
tail  began  to  be  most  strongly  marked,  several  short 
streamers  could  be  seen ;  and  occasionally  there 
seemed  to  be  transverse  bands  in  the  tail,  nearly  a 
half  a  degree  in  breadth,  with  clear,  well-defined 


GrREAT   COMET   OF   1858. 


139 


outlines,  and  closely  resembling  the  auroral  stream- 
ers, with  the  exception  only  that  they  retained  their 
relative  positions  in  the  tail.  On  the  10th  of  Octo- 
ber, the  date  on  which  the  tail  had  attained  its 
greatest  dimensions,  these  slightly  diverging  bands, 
—  the  point  of  divergence  being  situated  between 
the  sun  and  the  nucleus,  —  alternating  with  dark 
spaces,  could  be  readily  distinguished,  and  in  reality 
gave  the  comet  a  most  beautiful  appearance.  They 
were,  on  an  average,  five  degrees  in  length,  and 
from  a  third  to  half  a  degree  in  breadth.  The  oppo- 
site diagram  represents  the  comet  as  it  appeared  at 
this  time. 

The  apparent  length  of  the  tail  was  necessarily 
affected  by  the  moonlight  which  prevailed  at  two 
different  times  during  the  period  of  visibility,  and 
we  are  therefore  unable  to  determine  the  precise 
variations  in  its  length  from  day  to  day.  The  fol- 
lowing gives  the  length  of  the  tail  for  successive 
dates,  as  observed  at  the  Observatory  at  Cambridge, 
Massachusetts : 


Date                             Length  of  tail 
Date<                               in  miles. 

Date. 

Length  of  tail 
in  miles. 

ug.  29,  1858    .     .  14,000,000 

Sept.  30,  1858   . 

.  26,000,000 

apt.    8, 

'      .     .  16,000,000 

Oct.      2,     "      . 

.  27,000,000 

12, 

.     .  19,000,000 

'        5,     "      . 

.  33,000,000 

17, 

.     .  10,000,000 

6,     "      . 

.  45,000,000 

23, 

.     .  12,000,000 

'        8,     " 

.  43,000,000 

24, 

.     .  12,000,000 

'       10,     "      . 

.  51,000,000 

25, 

.     .  17,000,000 

'       12,     "      . 

.  39,000,000 

27, 

.     .  18,000,000 

'       15,     «      . 

.  14,000,000 

28, 

.    .  26,000,000 

140  TREATISE   ON    COMETS. 

The  discrepancy  which  seems  to  exist  in  the  mea- 
surements from  the  17th  to  the  25th  of  September, 
inclusive,  is  to  be  attributed  to  the  influence  of  the 
moonlight ;  and  any  other  slight  variation  from  a 
regular  increase  which  may  be  apparent,  is  probably 
due  to  a  hazy  or  unfavorable  state  of  the  atmo- 
sphere at  the  time  when  the  measurements  were 
taken . 

The  breadth  of  the  tail  at  the  extremity  was 
found,  on  the  25th  of  September,  to  be  1,500,000 
miles.  On  the  30th  it  was  3,000,000  miles,  and  on 
the  10th  of  October  10,000,000  miles.  The  stream- 
ers were  from  10,000,000  to  50,000,000  miles  in 
length ;  and  in  one  instance,  at  least,  their  breadth 
at  the  extremity  was  upwards  of  1,000,000  miles. 

The  nucleus  of  the  comet  was  observed  to  undergo 
the  most  rapid  changes  that  can  be  readily  conceived 
of,  not  only  in  form,  but  also  in  size.  On  the  19th 
of  July,  its  diameter  was  found  to  be  5600  miles ; 
August  30th,  it  was  4660  miles ;  and  September 
24th  it  was  only  1030  miles.  On  the  5th  of  October, 
it  was  certainly  less  than  540  miles  in  diameter; 
while  on  October  18th  it  had  increased  to  900  miles. 
On  the  8th  of  September,  the  diameter  of  the  nu- 
cleus was  ascertained  to  be  2000  miles ;  and  in  im- 
mediate contact  with  it  was  an  intensely  brilliant 
nebulosity,  having  a  diameter  of  about  3000  miles, 
while  the  diffused  light  was  estimated  to  extend 


G  n  B  A  T    COMET    OF   1858.  141 

40,000  or  50,000  miles  in  the  direction  of  the  sun. 
About  two  weeks  later,  it  was  noticed  that  there 
was  interposed  between  the  nucleus  and  the  sun,  an 
obscure,  crescent-shaped  outline,  within  which  the 
light  was  unequally  distributed,  and  which  had  a 
strangely  confused  and  chaotic  look.  The  nucleus 
at  this  time  was  singularly  brilliant,  and  appeared 
elongated  on  the  upper  side — a  phenomenon  which 
was  supposed  to  indicate  the  existence  of  some  in- 
ternal disturbing  force.  Outside  of  the  nucleus 
there  was  a  bright  envelope,  whose  vertex  was  in 
the  direction  of  the  sun,  and  about  6000  miles  dis- 
tant from  the  nucleus.  This  envelope  was  sur- 
rounded on  its  outer  margin  by  a  dark  band,  sepa- 
rating it  from  a  second  and  less  brilliant  envelope, 
whose  vertex  was  distant  from  that  of  the  first  one 
about  7500  miles.  This  again  was  encircled  by  a 
dark  arch,  outside  of  which  there  was  an  atmosphere 
of  faint  and  diffused  nebulosity,  which  gradually 
faded  away,  and  became  invisible  at  a  distance  of 
about  40,000  miles  from  the  nucleus.  These  bands 
could  be  traced  through  an  arc  of  more  than  200 
degrees,  around  the  nucleus ;  but  extended  further 
into  the  train  on  the  brighter  or  upper  side  with 
reference  to  the  horizon. 

The  phenomena  which  these  successive  bands  or 
envelopes  presented,  was  of  the  most  instructive 
and  interesting  character.  They  were  observed  in 


142  TREATISE   ON    COMETS. 

succession  to  disengage  themselves  from  the  nucleus, 
then  gradually  to  expand  and  recede,  and  finally  to 
disappear  in  the  formation  of  the  tail.  So  plainly 
was  this  exhibited,  that  each  envelope  could  be 
traced  through  all  its  successive  stages ;  and  thus 
have  the  means  been  furnished  for  investigating  in 
detail  the  mysterious  processes  by  which  the  train 
is  thrown  out  from  the  nucleus,  under  the  action  of 
forces  whose  nature  and  mode  of  operation  will  be 
fully  explained  when  we  come  to  treat  of  the  theory 
of  the  physical  constitution  of  comets,  and  of  the 
formation  of  their  tails.  In  this  way,  also,  we  are 
enabled  to  account  for  the  rapid  decrease  of  the 
diameter  of  the  nucleus  as  the  comet  approached 
the  sun,  when  the  matter  of  which  it  is  composed 
was  driven  off  to  form  the  tarl ;  and,  also,  the  in- 
crease in  the  size  of  the  nucleus  as  the  comet  again 
receded.  On  the  3d  of  October,  the  nucleus  ap- 
peared to  be  divided  into  two  distinct  parts, —  one, 
however,  being  much  brighter  than  the  other. 
These  were  separated  by  a  distinctly  dark  opening, 
and  were  distant  about  4000  miles.  No  change  was 
observed  at  this  time  in  the  relative  positions  and 
form  of  the  envelopes,  although  the  tail  seemed  to 
have  been  sensibly  affected  —  the  bifurcation  being 
much  more  distinct,  and  broader  in  comparison  with 
the  breadth  of  the  tail. 

The  opposite  diagram  is  a  telescopic  view  of  the 


GREAT  COMET  OF  1858.      143 

nucleus  and  part  of  the  tail  of  the  comet  on  the  3d 
of  October,  1858. 

The  successive  envelopes  are  here  exhibited  pre- 
cisely as  they  appeared  when  the  comet  was  seen 
through  a  powerful  telescope.  The  division  of  the 
nucleus  is  also  indicated,  and  the  dark  space  sepa- 
rating the  tail  into  two  parts.  The  development  of 
these  envelopes,  as  already  remarked,  was  extremely 
interesting.  In  most  cases  they  were  observed  to 
escape  from  the  vertex  of  the  nucleus ;  but  this  was 
by  no  means  general.  They  were  also  seen  escaping 
from  the  sides  of  the  nucleus,  and  have  been  de- 
scribed as  an  escape  of  jets  of  luminous  gas,  which 
streamed  off  like  light  spray  thrown  up  against  an 
opposing  wind  and  driven  before  it  —  a  description 
which  is  certainly  very  aptly  conceived.  The  entire 
distance  between  the  vertex  of  the  nucleus  and  the 
inner  surface  of  the  outer  envelope,  varied  from  ten 
to  twenty  thousand  miles.  It  was  noticed,  also,  that 
just  before  an  eruption  took  place,  the  brilliancy  of 
the  nucleus  was  perceptibly  greater  than  either  im- 
mediately before  or  after  that  event ;  and  the  varia- 
tions were  so  rapid  as  to  indicate  almost  the  exact 
instant  at  which  the  eruption  took  place.  In  order 
to  show  the  progressive  motion  of  the  envelopes 
from  their  point  of  origin,  the  following  distances 
from  the  nucleus  to  the  vertex  of  one  of  these  enve- 
lopes, tor  each  day  during  the  period  of  its  trans- 


144 


TREATISE   ON    COMETS. 


formation,    as   observed  by  Bond,   at   Cambridge, 
Mass.,  are  given: 


Date. 

Distance  of  Vertex 
from  nucleus 
in  miles. 

Date. 

Distance  of  Vertex 
from  nucleus 
in  miles. 

1858,  Sept.  27, 

.     .     .    3,500 

1858,  Oct.  6, 

.     .     .     10,100 

"        "     29, 

.     .     .    6,000 

"    8, 

.     .     .     12,400 

"     Oct.     2, 

.     .     .    7,500 

"       "    9, 

.     .     .     13,200 

"       4, 

.     .     .    8,900 

"  10, 

.     .     .     14,100 

"       5, 

.     .     .    9,550 

It  appears,  therefore,  that  the  daily  motion  of 
the  vertex  of  the  envelope,  relative  to  the  vertex 
of  the  nucleus,  varied  from  300  to  1200  miles. 
This  rate  of  motion  was  found  to  coincide  very 
closely  with  that  of  the  other  envelopes  at  the  same 
date.  The  motion  of  the  envelopes,  it  might  fur- 
ther be  remarked,  varied  with  the  position  of  the 
cornet  with  respect  to  the  sun.  The  manner  in 
which  these  envelopes  were  driven  off,  and  the  evi- 
dent tendency  of  each  towards  a  condensation 
around  a  central  axis,  together  with  the  fact  that 
the  axes  manifested  a  disposition  to  diverge  from 
the  sun,  has  led  to  the  conjecture  that  these  collec- 
tions of  nebulosity  were  in  reality  a  group  of  new 
comets  in  process  of  formation.  It  is  therefore 
unfortunate,  that  the  circumstances  were  such  as  to 
preclude  the  possibility  of  following  them  to  a  more 
complete  development. 

We  may  thus  comprehend  the  peculiar  interest 
which  attaches  to  the  great  comet  of  1858.  The 


GREAT   COMET   or   1858.  145 

long  period  during  which  it  was  visible  before  it  had 
attained  its  most  magnificent  proportions,  had  ena- 
bled astronomers  to  be  prepared  to  observe  all  the 
phenomena  of  the  changes  of  its  physical  appear- 
ance with  the  greatest  facility.  Other  comets  have 
appeared  which  furnished  phenomena  of  precisely 
the  same  character,  and  perhaps  on  as  grand  a  scale, 
but  the  state  of  astronomical  science  was  such  that 
their  importance  was  not  fully  understood,  and  con- 
sequently the  observations  were  not  so  refined  and 
extensive.  Under  such  circumstances,  we  may  de- 
clare, without  hesitation,  that  the  comet  now  under 
consideration  has  been  of  more  essential  service  to 
science,  and  otherwise  of  more  universal  interest, 
than  any  other  which  has  ever  been  seen.  The 
splendor  of  its  appearance  in  October  elicited  the 
most  profound  admiration  throughout  the  civilized 
world;  and  although,  like  its  predecessors,  it  was 
assigned  its  share  in  the  cause  of  any  unusual  events 
which  may  have  taken  place  simultaneously  with 
its  appearance,  yet  it  is  certain  that  the  general 
diffusion  of  knowledge  which  characterizes  the 
present  age,  operated  very  effectually  to  counteract 
all  superstitious  fears,  which,  otherwise,  it  would  not 
have  failed  to  excite. 

The  computations  which  have  been  made  respect- 
ing its  orbit,  show  that  the  period  of  its  revolution 
is  about  2000  years.     The  following  are  some  of 
13  K 


146  TREATISE   ON    COMETS. 

the  results  which  have  been  obtained  by  different 
astronomers : 

Bruhns, 2102  years. 

Watson, 2415       " 

Lowy, 2495      " 

Brlinnow, 2470       " 

Newcomb. 1854      " 

The  uncertainty  of  the  period  arises  from  the 
great  eccentricity  of  the  orbit ;  and  since  the  obser- 
vations made  in  the  southern  hemisphere  during 
several  months  after  its  disappearance  in  the  north- 
ern, have  not  been  employed  in  the  determination 
of  these  results,  this  uncertainty  may  be  materially 
reduced  in  subsequent  computations.  The  disturb- 
ance produced  in  its  motion  by  the  attraction  of  the 
planets  during  the  period  of  its  visibility,  have  also 
been  neglected,  the  introduction  of  which  may  affect 
the  results  slightly.  It  is  probable,  however,  that  a 
more  complete  investigation  will  give  a  period  dif- 
fering but  little  from  2400  years.  The  comet  must 
therefore  be  distant  from  the  sun,  when  in  the  aphe- 
lion or  most  remote  part  of  its  orbit,  no  less  than 
34,000,000,000  of  miles,  or  about  one  six-hundredth 
part  of  the  distance  of  the  nearest  fixed  star.  Great 
as  is  this  distance,  the  comet  will  still  be  obedient 
to  the  controlling  influence  of  the  great  central 
body  of  our  system  ;  and  after  having  performed  its 
long  journey  of  more  than  two  thousand  years,  it 


NUMBER   OF   COMETS.  147 

will  again  return  to  greet  the  inhabitants  of  our 
earth. 

We  have  thus  briefly  described  some  of  the  most 
remarkable  comets  which  have  appeared  during  the 
period  of  authentic  history.  To  notice  all  which 
appeared  would  not  only  have  been  tedious  and 
uninteresting,  but  also  would  have  required  more 
space  than  could  properly  have  been  devoted  to  this 
branch  of  our  subject.  The  number  of  comets  which 
have  appeared  since  the  birth  of  Christ,  in  each 
successive  century,  is  as  follows  :  First  century,  22  ; 
second,  23 ;  third,  44 ;  fourth,  27 ;  fifth,  16 ;  sixth, 
25  ;  seventh,  22  ;  eighth,  16  ;  ninth,  42  ;  tenth,  26  ; 
eleventh,  36;  twelfth,  26:  thirteenth,  26;  four- 
teenth, 29;  fifteenth,  27;  sixteenth,  31;  seven- 
teenth, 25;  eighteenth,  64;  nineteenth  (till  1860), 
114.  Many  of  these  have  been  reappearances  of 
the  same  comet;  and  although  the  total  number 
here  given  is  651,  yet  we  are  not  to  understand  that 
there  has  been  651  different  comets  observed  during 
this  time.  Of  these,  we  have  described  those  which 
were  remarkable  for  their  brilliancy,  and  whose 
periods  of  revolution  have  not  been  exactly  deter- 
mined. The  periodic  comets,  properly  so  called, 
will  be  described  when  we  shall  have  explained  the 
manner  in  which  the  orbits  of  these  bodies  are 
determined. 

The  number  of  comets  here  given  comprises  only 


148  TREATISE   ON   COMETS. 

those  whose  appearances  have  actually  been  re- 
corded, while  it  is  perfectly  certain  that  many  have 
visited  the  sun  unperceived.  By  comparing  the 
positions  of  the  orbits  of  those  comets  whose  ele- 
ments have  been  determined,  it  is  found  that,  unlike 
the  planets,  they  are  distributed  over  the  entire 
surface  of  the  heavens,  without  preference  of  any 
one  region  to  any  other.  Again,  the  points  where 
they  pass  through  the  plane  of  the  earth's  orbit,  are 
found  to  be  uniformly  distributed  in  every  direction 
round  the  sun.  The  points  where  they  pass  nearest 
the  sun  are  also  distributed  uniformly  round  that 
body,  and  their  perihelion  distances  are  such  as 
leads  to  the  supposition  of  their  uniform  distribution 
through  space.  If  now  we  assume  this  to  be  actu- 
ally the  case,  since  these  bodies  can  only  be  seen 
from  the  earth  when  within  the  orbit  of  Mars,  we 
may  be  able  to  form  an  estimate  of  the  probable 
number  of  comets  in  space.  From  considerations 
such  as  these,  and  by  a  process  of  reasoning  which 
it  is  not  necessary  to  explain,  Arago  concludes  that 
at  least  seven  millions  of  comets  are  enclosed  within 
the  limits  of  our  system.  But  a  small  portion  of 
these,  comparatively,  can  ever  be  seen  from  the 
earth ;  and  although  this  hypothesis  may  seem  un- 
reasonable, yet  there  is  every  reason  to  regard  it  as 
a  very  approximate  estimate  of  the  number  of  these 
wonderful  bodies. 


ORBITS   OF   COMETS.  149 


CHAPTER   III. 

ORBITS   OF    COMETS COMPUTATION    OF  AN  ORBIT  —  EPHEMERIS  OP  A 

COMET — PERIODIC  COMETS HALLEY's  COMET  —  ENCKE's  COMET 

BIELA'S  COMET  —  FATE'S  COMET  —  DE  vico's  COMET  —  BRONSEN'S 
COMET — D'ARREST'S  COMET — TUTTLE'S  COMET — WINNECKE'S  COMET 
—  LEXELL'S  COMET  OF  1770 — OTHER  PERIODIC  COMETS — RELATION 
SUPPOSED  TO  EXIST  BETWEEN  THE  COMETS  OF  SHORT  PERIOD  AND 

THE  ASTEROID  PLANETS POSSIBILITY  OF  A  COLLISION  BETWEEN 

A  COMET  AND  PLANET PROBABLE  APPULSE  OF  TWO  COMETS. 

As  soon  as  it  was  ascertained  that  the  comets  have 
no  connection  with  the  earth,  but  move  in  the  region 
of  the  planets,  the  investigation  of  their  orbits,  and 
their  relation  to  the  other  known  bodies  of  the  uni- 
verse, became  a  problem  of  the  very  highest  interest. 
The  illustrious  Kepler  endeavored  to  represent  their 
observed  places,  by  supposing  them  to  move  in 
straight  lines  with  a  variable  velocity — a  supposition 
which  was  subsequently  adopted  by  several  astron- 
omers ;  and  computations  were  made,  in  accordance 
with  it,  respecting  the  motions  of  some  of  those 
comets  which  had  previously  appeared.  This  hypo- 
thesis of  Kepler,  as  we  shall  see,  is  now  known  to 
be  unfounded,  yet  the  real  paths  pursued  by  the 
comets  are  such  that,  for  a  considerable  distance, 
they  very  nearly  coincide  with  a  straight  line ;  and 
13* 


150  THEATISE   ON    COMETS. 

we  may  therefore  understand  how  Cassini  was  ena- 
bled to  predict  the  places  of  a  comet,  for  a  few 
weeks  in  advance,  with  considerable  precision. 
Hevelius,  a  few  years  later,  found,  by  observing  the 
comet  of  1665,  that  its  path  was  curvilinear,  and 
that  the  orbit  might  be  a  parabola.  He  did  not, 
however,  conjecture  what  would  be  the  position  of 
the  sun  with  respect  to  the  curve ;  and  it  was  not 
till  the  appearance  of  the  great  comet  of  1680,  that 
this  question  was  definitely  settled.  It^  was  then 
determined,  by  Dorfel  and  Newton,  that  the  orbit  of 
the  comet  was  a  parabola,  with  the  sun  situated  in 
a  fixed  point  within  the  curve,  called  the  focus.  It- 
was  further  determined  by  Newton,  that  the  comets 
form  a  part  of  the  solar  system ;  that  they  describe 
orbits  around  the  sun  in  obedience  to  his  attractive 
force  ;  and  that  their  motions  are  in  accordance  with 
the  same  laws  which  govern  the  motions  of  the 
planets.  He  also  explained  a  method  of  determining 
the  orbit  from  positions  of  the  comet  observed  at 
the  earth.  This  was  the  first  attempt  to  reduce  the 
motions  of  the  comets  to  numerical  computation, 
in  a  manner  which  could  be  fully  investigated  by 
mathematical  analysis. 

In  order  to  effect  a  determination  of  the  size  and 
position  of  the  orbit  of  a  comet,  Newton  proposed 
two  different  methods.  In  the  first  he  supposes 
that  a  small  portion  of  the  orbit  may  be  regarded 


COMPUTATION   OF  AN   ORBIT.         151 

as  a  straight  line,  described  with  a  uniform  motion, 
and  that  its  segments,  intercepted  by  straight  lines 
drawn  from  the  earth  to  the  comet,  at  the  times  of 
the  respective  observations,  are  proportional  to  the 
intervals  of  time  between  these  observations.  In 
this  manner  the  ratio  of  these  segments  becomes 
known  ;  and  since  the  portion  of  the  orbit  included 
between  the  extreme  observations  is  a  straight  line, 
its  projection  on  the  plane  of  the  earth's  orbit  is 
also  a  straight  line,  divided  similarly  to  the  orbit 
itself  by  the  projections  of  the  straight  lines  joining 
the  places  of  the  earth  and  the  comet.  It  would 
seem,  therefore,  to  be  a  very  simple  process  to  find 
the  projected  orbit;  but  it  happens,  unfortunately, 
that  this  method  is  one  which,  in  the  case  of  the 
orbit  of  a  comet,  admits  of  almost  an  infinite  num- 
ber of  solutions,  and,  consequently,  is  so  nearly 
indeterminate  that  no  definite  results  can  be  ob- 
tained. The  other  method  proposed  by  Newton 
was  more  general  and  complete;  and,  as  far  as 
theory  is  concerned,  is  wholly  unexceptionable.  It 
is  found,  however,  that  when  actually  applied  in  the 
determination  of  an  orbit,  it  leads  to  computations 
excessively  complicated  and  laborious,  and  conse- 
quently has  never  been  generally  adopted. 

The  theory  of  the  determination  of  the  orbits  of 
comets  remained  for  many  years  where  Newton  had 
left  it ;  arid  astronomers  were  still  attempting,  but 


152  TKEATISE   ON   COMETS. 

in  vain,  to  discover  a  method  by  which  an  orbit 
might  be  determined  with  accuracy,  without  making 
it  necessary  to  have  recourse  to  such  extended  nu- 
merical calculations.  Failing  in  every  attempt  at  a 
direct  solution,  they  were  compelled  to  adopt  a 
method  of  trial  and  error,  and  thus,  by  successive 
hypotheses,  to  approximate  finally  to  an  orbit  which 
would  nearly  represent  the  observations.  Bosco- 
vich,  however,  undertook  to  solve  this  difficult  pro- 
blem directly ;  and  the  solution  which  he  gave  is 
remarkable  as  being  the  first  in  \vhich  the  velocity 
of  the  comet  in  its  orbit  was  regarded  as  one  of  the 
essential  conditions.  His  method,  as  might  be  ex- 
pected, is  excessively  complicated ;  and  from  the 
multiplicity  of  both  algebraical  formulae  and  gra- 
phical operations  which  it  presents,  it  has  never 
been  employed  in  the  actual  computation  of  an 
orbit. 

The  next  attempt  to  solve  this  problem  was  made 
by  Lambert ;  and  among  the  formulae  which  he  ob- 
tained relative  to  the  motion  of  a  body  in  a  parabola, 
there  is  one  extremely  remarkable,  on  account  of  its 
elegance,  which  gives  the  time  of  describing  an  arc 
of  the  orbit  in  terms  of  its  chord  and  the  two  lines 
drawn  from  its  extremity  to  the  centre  of  the  sun. 
This  formula  is  still  employed  in  every  method 
which  is  in  use  for  the  solution  of  this  problem,  since 
it  affords  a  very  easy  means  of  correcting  the  hypo- 


COMPUTATION   OF   AN   ORBIT.         153 

thetical  values  which  it  is  necessary  to  substitute  in 
the  equations  of  the  problem.    He  also  showed  how 
it  may  be  determined  at  once  whether  the  distance 
of  the  comet  from  the  sun  is  greater  or  less  than  the 
distance  of  the  earth  from  the  sun,  thus  enabling 
the  computer  to  abridge,  very  materially,  the  pre- 
liminary computations.     The  method  of  Lambert 
was  subsequently  modified  by  the  researches  of  La- 
grange  and  Laplace ;  and  the  results  finally  arrived 
at  by  these  illustrious  geometers  are  now  adopted, 
for  the  most  part,  as  the  basis  of  all  analytical  solu- 
tions.   Still  another  method  was  proposed  by  Gibers 
for  finding  the  orbit  of  a  comet  moving  in  a  parabola, 
in  which,  as  in  the  case  of  those  last  mentioned, 
only  three  different  places,  as  seen  from  the  earth, 
are  required.     He  supposes  that  the  chord  which 
joins  the  places  of  the  comet  at  the  epoch  of  the 
first  and  third  observations,  is  divided  by  the  line 
drawn  from  the  comet  to  the  sun,  at  the  time  of  the 
second  observation,  into  segments  proportional  to 
the   intervals   of   time   between   the   observations. 
This  method  is  the  simplest  and  most  direct  which 
has  been  devised ;  but  gives  accurate  results  only 
when  the  intervals  of  time  betwen  the  dates  of  the 
observations  are  very  nearly  equal.     In  those  cases 
where  the  inequality  of  the  intervals  is  considerable, 
and  especially  when  the  comet  is  further  from  the 
sun  than  the  earth,  it  gives  very  unsatisfactory  re- 


154  TREATISE   ON    COMETS. 

suits ;  and  for  this  reason  Legendre  has  devised  a 
method  suitable  to  all  such  cases,  in  which — instead 
of  assuming  that  the  chord  of  the  arc  of  the  orbit 
between  the  first  and  third  observations  is  divided 
by  a  line  drawn  from  the  comet  to  the  sun  at  the 
time  of  the  second  observation,  in  the  ratio  of  the 
elapsed  times — he  finds  an  expression  for  the  value 
of  each  of  the  segments  in  terms  of  the  time.  This 
last  method  may  be  employed  with  success  in  cases 
where  that  of  Gibers  fails,  but  the  numerical  calcu- 
lations are  much  more  complicated  and  tedious. 

We  have  thus  stated,  somewhat  in  detail,  some  of 
the  various  methods  which  have  been  devised  for  the 
solution  of  the  problem  which  requires  the  orbit  of 
a  comet  to  be  determined  by  means  of  its  successive 
positions  observed  from  the  earth ;  and  what  is  here 
given  will  be  sufficient  to  show  that  it  is  one  of  the 
most  difficult  problems  of  astronomy.  If  we  already 
knew,  and  could  subject  to  calculation,  the  causes 
which  originally  determined  the  motions  of  the 
heavenly  bodies,  we  could  assign,  at  once,  the  data 
for  a  complete  solution  of  this  problem  without  the 
aid  of  observation.  But  as  these  are,  and  must  ever 
remain,  unknown,  we  have  no  means  of  arriving  at 
the  desired  results,  except  by  the  method  just  ex- 
plained. Again,  if  the  observer  could  be  situated 
at  the  centre  of  the  sun,  the  real  and  apparent  mo- 
tions of  the  comet  would  be  identical,  and  the  deter- 


COMPUTATION   OF   AN   ORBIT.         155 

urination  of  its  orbit  would  be  a  process  extremely 
simple.  But  since  the  observer  is  stationed  on  the 
earth,  which  is  itself  in  motion  in  an  eccentric  orbit, 
it  may  readily  be  perceived  that  it  is  necessary  to 
refer  all  positions,  as  seen  from  the  centre  of  the 
earth,  to  the  corresponding  places  referred  to  the 
centre  of  the  sun,  in  order  to  arrive  at  the  desired 
result,  and  this  it  is  which  makes  the  problem 
difficult. 

We  have  already  remarked  that  it  was  discovered 
by  Newton  that  the  comets  were  obedient  to  the 
same  laws  which  regulate  the  motions  of  the  planets 
belonging  to  the  solar  system ;  and  we  are,  therefore, 
enabled  to  adopt  in  this  case,  also,  those  beautiful 
laws  of  planetary  motion  discovered  by  Kepler. 
These  laws  are  three  in  number,  and  are  as  follows : 
1st.  The  heavenly  bodies  revolve  around  the  sun  in 
conic  sections  whose  common  focus  is  the  sun ;  2d. 
The  radius-vector,  or  line  joining  the  planet  or 
comet  with  the  sun,  describes  equal  areas  in  equal 
times,  and,  consequently,  in  unequal  times  areas 
proportional  to  the  times ;  3d.  The  squares  of  the 
periodic  times  are  to  each  other  as  the  cubes  of  the 
mean  distances  of  the  bodies  from  the  sun.  These 
three  laws,  the  direct  result  of  the  law  of  universal 
gravitation,  together  with  three  observed  positions 
of  the  comet,  are  all  the  data  required  in  finding  the 
magnitude  and  position  of  its  orbit,  and  its  position 


156  TREATISE   ON    COMETS. 

in  the  orbit  at  any  given  time.  But  we  have  here 
spoken  of  a  class  of  curves  called  conic  sections,  and 
it  may  not  therefore  be  improper  to  explain  their 
character  in  this  connection. 

If  we  take  a  cone — which  may  be  conceived  of  as 
being  a  solid  formed  by  the  revolution  of  a  right- 
angled  triangle  around  the  perpendicular  side  as  a 
fixed  axis  —  and  pass  planes  through  it  at  different 
degrees  of  inclination  with  respect  to  the  axis,  the 
intersections  of  these  planes  with  the  surface  of  the 
cone  will  give  us  one  or  all  of  the  three  curves  known 
as  conic  sections;  namely,  the  ellipse,  parabola,  and 
hyperbola.  The  plane  which  passes  through  the  cone 
so  as  not  to  cut  the  base,  but  only  the  slant  surface, 
gives  an  ellipse,  a  curve  which,  next  to  the  circle,  is 
the  most  simple.  Those  which  cut  the  base  will 
mark  out  on  the  slant  surface  of  the  cone  segments 
of  either  the  parabola  or  hyperbola,  depending 
simply  on  the  position  of  the  plane  with  reference 
to  the  axis.  This  explains  why  these  curves  are 
termed  conic  sections ;  but  their  true  character  will 
be  much  more  readily  comprehended  by  a  more 
familiar  illustration. 

If  we  take  a  fine  thread  and  fasten  its  extremities 
at  two  points  on  a  plane  surface,  the  distance  be- 
tween these  points  being  less  than  the  length  of  the 
thread  —  which  will  thus  lie  loosely  between  the 
points  —  and  if  now  this  thread  is  kept  tightly 


COMPUTATION   OF  AN   ORBIT.        157 

stretched  by  means  of  a  pencil  which  is  made  to 
move  round  the  entire  circumference  permitted  by 
the  looseness  of  the  thread,  the  curve  thus  described 
will  be  an  ellipse,  or,  as  it  is  sometimes  called,  an 
oval.  Now  it  is  easy  to  be  seen  that  since  the  thread 
has  retained  the  same  length,  and  has  been  kept 
tightly  drawn  out  during  the  description  of  the 
curve,  the  sum  of  the  lines  drawn  from  any  point 
of  the  curve  to  the  two  points  at  which  the  thread 
was  fastened,  will  be  the  same  for  every  such  point 
in  the  curve.  The  two  points  within  the  curve  at 
which  the  ends  of  the  thread  were  fixed,  are  called 
the  foci  of  the  ellipse,  and  in  the  case  of  the  elliptic 
orbit  of  a  comet  the  sun  is  placed  in  one  of  these 
points. 

Let  us  now  suppose  that  the  distance  between  these 
points  is  increased,  the  length  of  thread  remaining 
the  same ;  and,  as  may  be  readily  seen  by  experiment, 
we  shall  obtain  a  curve  similar  to  the  one  already 
traced,  only  that  its  form  has  been  altered,  while  the 
length  of  the  line  joining  any  point  in  the  circum- 
ference with  the  new  foci,  remains  as  in  the  previous 
case.  By  continuing  to  increase  the  distance  be- 
tween the  foci,  using  always  the  same  thread,  we 
shall  find  that  the  successive  ellipses  will  be  more 
and  more  flattened  or  elongated,  until  finally  they 
will  differ  but  little  from  a  straight  line.  If,  on  the 
contrary,  the  distance  between  the  foci  is  successively 
14 


158  TREATISE   ON    COMETS. 

shortened,  the  ellipses  formed  will  gradually  become 
less  and  less  flattened,  and  will  scarcely  differ  from 
a  circle.  If  the  foci  be  made  to  coincide,  then  will 
the  curve  traced  be  a  circle,  and  for  this  reason  a 
circle  is  sometimes  said  to  be  a  species  of  ellipse. 
The  line  which  is  drawn  through  the  foci  and  termi- 
nating in  the  curve  at  either  extremity,  is  called  the 
major  axis  of  the  ellipse ;  while  that  which  bisects 
this  at  right  angles,  terminating  also  in  the  curve  at 
opposite  sides,  is  called  the  minor  axis.  The  ratio 
of  the  major  axis  to  the  line  joining  the  foci  is  called 
the  eccentricity.  The  eccentricity  will  therefore  be 
greatest  when  the  foci  are  at  their  greatest  distance, 
and  least  when  they  coincide,  as  in  the  case  of  the 
circle. 

Having  thus  explained  the  method  of  projecting 
an  ellipse,  and  the  general  properties  of  the  curve, 
we  will  proceed  to  illustrate  the  construction  of  a 
parabola.  For  this  purpose  let  a  ruler  be  placed  in 
any  desired  position  on  a  plane  surface,  and  remain 
fixed  in  that  position.  Another  ruler  is  now  placed 
at  right  angles  to  this,  but  capable  of  being  moved 
along  it.  The  latter  must  be  so  arranged,  that  in 
moving  along  the  former  they  shall  always  form  a 
right  angle ;  or,  in  other  wrords,  one  shall  continue 
perpendicular  to  the  other.  Let  us  now  take  a  string 
equal  in  length  to  the  perpendicular  edge  of  the 
movable  ruler,  and  fix  one  end  of  this  string  at  any 


COMPUTATION   OF   AN   ORBIT.         159 

point  in  the  plane  surface  on  the  same  side  of  the 
fixed  ruler  on  which  the  movable  one  is  placed,  the 
other  end  being  attached  to  the  latter  at  the  outer 
extremity  of  the  perpendicular  edge.  If,  then,  by 
means  of  a  pencil,  we  commence  at  the  point  where 
the  string  is  fastened  to  the  movable  ruler,  and  move 
it  slowly  towards  the  fixed  ruler,  always  keeping  the 
string  close  to  the  edge  of  the  former,  and  at  the 
same  time  move  the  latter  along  it  in  the  direction 
of  the  fixed  end  of  the  string  —  passing  this  point, 
and  continuing  until  the  pencil  arrives  again  at  the 
other  extremity  of  the  string,  the  curve  thus  traced 
will  be  a  parabola.  By  varying  the  distance  of  the 
fixed  point  of  the  string  from  the  fixed  ruler,  a  great 
variety  of  parabolas  may  be  traced  out.  The  fixed 
ruler  is  called  the  directrix  of  the  parabola,  and  the 
point  within  the  curve  at  which  the  string  is  made 
stationary,  is  called  the  focus.  In  the  case  of  the 
orbits  of  the  comets,  the  sun  is  placed  in  this,  the 
only  focus  of  the  parabola.  It  may  thus  be  per- 
ceived that  the  parabola  is  constructed  in  such  a 
manner,  that  the  distance  between  the  focus  and  any 
point  of  the  curve  is  equal  to  the  perpendicular  dis- 
tance between  the  same  point  and  the  directrix. 

]N~ext,  to  describe  an  hyperbola.  On  a  plane  sur- 
face draw  two  straight  lines  at  right  angles  and  bi- 
secting each  other,  the  horizontal  line,  however, 
being  the  longest.  Then  take  two  points  on  the 


160  TREATISE   ON    COMETS. 

longest  of  these  lines,  one  on  each  side  of  the  per- 
dicular  line  and  at  equal  distances  from  it.  Place  a 
ruler  so  that  one  end  of  it  may  revolve  around  one 
of  these  points  —  being  kept  closely  pressed  to  the 
plane  surface — and  to  the  other  end  attach  a  string, 
which  is  also  to  be  fastened  at  the  other  extremity 
to  the  fixed  point  taken  on  the  opposite  side  of  the 
perpendicular  line  from  that  around  which  the  ruler 
revolves.  The  ruler  is  now  revolved  slowly  around 
its  axis,  while  the  string  is  stretched  by  means  of  a 
pencil,  commencing  at  the  point  where  it  is  fastened 
to  the  ruler,  and  moving  it  towards  the  other  end  of 
the  same,  until  it  finally  passes  the  horizontal  line, 
when  it  will  move  back  again  along  the  ruler  to  the 
point  of  beginning.  The  string  must  be  kept  close 
to  the  edge  of  the  ruler  at  the  point  where  the  pen- 
cil is  placed,  and  in  the  same  manner  for  every  point 
in  the  curve  which  will  thus  be  traced  out  on  the  plane 
surface,  and  which  is  called  an  hyperbola.  The 
fixed  points  which  have  been  taken  in  the  horizontal 
line  are  called  the  foci  of  the  hyperbola,  and  in  the 
case  of  the  orbit  of  a  comet  moving  in  an  hyperbola, 
the  sun  is  placed  in  the  focus  which  is  within  the 
curve.  The  point  at  which  the  fixed  lines  first  drawn 
intersect,  is  called  the  centre  of  the  hyperbola.  The 
point  where  the  curve  crosses  the  horizontal  line  is 
called  the  vertex  of  the  hyperbola,  and  the  distance 


COMPUTATION   OF   AN   ORBIT.         161 

between  the  vertex  and  the  point  of  intersection  of 
the  fixed  line,  is  called  the  semi-transverse  axis  of  the 
hyperbola.  That  part  of  the  perpendicular  line 
which  is  limited  by  an  arc  of  a  circle  described 
around  the  vertex  of  the  hyperbola  as  a  centre,  with 
a  radius  equal  to  the  distance  of  the  focus  from  the 
centre,  is  called  the  conjugate  axis  of  the  hyperbola. 
The  ratio  of  the  semi-transverse  axis  to  the  line 
joining  the  centre  with  the  focus ;  or,  which  is  the 
same,  the  quotient  which  arises  from  dividing  the 
latter  by  the  former,  is  called  the  eccentricity ;  and  it 
will  be  perceived,  from  what  has  just  been  stated, 
that  this  ratio  will  always  he  greater  than  unity. 
In  the  ellipse,  it  follows,  from  the  same  principles, 
that  this  ratio  or  quotient  resulting  from  a  division 
of  the  line  joining  the  foci,  by  the  major  axis,  will 
always  be  less  than  unity.  The  eccentricity  of  the 
ellipse  may,  therefore,  vary  between  the  limits  0  and 
1,  the  eccentricity  of  the  parabola  is  1,  while  that 
of  the  hyperbola  may  have  any  value  greater  than 
this. 

In  this  way  these  curves  are  found  to  bear  an  in- 
timate relation  to  each  other,  and  in  order  to  ex- 
hibit at  once  their  characteristic  differences,  together 
with  their  general  resemblance,  the  following  dia- 
gram is  given : 

14*  L 


162 


TREATISE   ON   COMETS 


The  distance  between  the  focus  and  vertex  is  the 
same  for  each  of  the  above  curves ;  and  this  dis- 
tance, in  the  case  of  the  orbit  of  a  comet,  is  called 
the  perihelion  distance. 

The  curves  which  we  have  just  described  we  have 
supposed  to  have  been  traced  out  with  a  short  piece 
of  thread ;  but  curves  may  be  imagined  as  being 
constructed  in  accordance  with  the  same  laws,  and 
having  therefore  the  same  properties,  traversing 
vast  and  inaccessible  regions  of  space,  and  in  which 
those  dimensions  which  we  have  represented  per- 
haps by  tenths  of  an  inch,  are  replaced  by  millions 
of  miles.  Such  are  the  orbits  of  the  comets ;  and 
in  one  of  these  curves  every  body  which  traverses 


COMPUTATION   OF  AN   ORBIT.         163 

our  system  is  constrained  to  move,  in  obedience  to 
the  law  of  universal  gravitation.  The  planets,  how- 
ever, although  permitted  by  that  law  to  move  in 
similar  paths,  are  found  to  revolve  around  the  sun 
in  orbits  which  are  purely  elliptical,  and  in  which 
the  eccentricity  is  generally  very  small.  They  are 
found  also  to  move  in  a  series  of  orbits  at  distances 
increasing  in  a  regular  progression,  confined  to 
limits  of  only  a  few  degrees  on  either  side  of  the 
orbit  of  the  earth,  and  in  the  same  direction  in  which 
the  earth  pursues  its  annual  course  around  the  sun. 
It  might  thus  seem  that  an  accordance  so  wonderful, 
and  an  order  so  admirable,  could  not  be  fortuitous ; 
and  not  being  enjoined  by  the  law  of  gravitation, 
must  be  ascribed  to  some  additional  character  apper- 
taining to  this  law,  with  which  we  are  unacquainted. 
But  the  comets  show  this  beautiful  law  exemplified 
and  obeyed  in  all  its  comprehensive  features ;  and 
thus  sweep  around  the  sun  in  all  the  varied  forms 
of  ellipses,  parabolas,  and  hyperbolas,  in  all  planes, 
at  all  distances,  and  indifferently  in  both  directions 
in  reference  to  the  motion  of  the  planets. 

The  comets  are  usually  seen  from  the  earth  when 
near  the  sun,  and  consequently  when  not  far  distant 
from  the  perihelia  of  their  orbits.  It  will  be  evi- 
dent, therefore,  by  an  inspection  of  the  diagram  on 
page  162,  that,  since  the  three  conic  sections  nearly 
coincide  for  a  considerable  distance  on  either  side 


164  TREATISE   ON   COMETS. 

of  the  vertex, — which  corresponds  to  the  perihelion 
of  the  orbit, — it  will  be  difficult  to  determine,  from 
the  small  segment  of  the  orbit  observed,  whether 
the  comet  moves  in  an  ellipse,  a  parabola,  or  an 
hyperbola.  If  the  centre  of  gravity  of  the  comet 
could  be  observed  with  the  utmost  precision,  this 
would  not  be  difficult;  but,  as  a  general  fact,  the 
observer  only  estimates  the  point  of  greatest  con- 
densation of  light,  which  he  assumes  to  be  the  cen- 
tre of  gravity,  and  the  place  of  which  in  the  heavens 
he  determines  by  reference  to  the  fixed  stars  in  its 
immediate  vicinity.  We  are  thus  left  in  doubt  in 
regard  to  the  exact  form  of  the  orbit,  unless  it  hap- 
pens that  the  comet  is  favorably  situated  for  its 
determination.  It  is  therefore  customary  among 
astronomers,  when  a  comet  has  made  its  appearance 
unpredicted,  to  compute  its  orbit  at  first  on  the  sup- 
position that  it  is  a  parabola;  and  then,  by  com- 
puting its  place  in  advance,  find  from  a  comparison 
of  the  actual  observations,  whether  this  hypothesis 
is  the  correct  one.  Should  it  be  found  to  be  impos- 
sible to  represent  the  observed  positions  of  the 
comet  by  a  parabola,  an  ellipse  is  next  computed ; 
and  when  this  also  fails,  recourse  is  had  to  the 
hyperbola,  which,  provided  the  previous  computa- 
tions are  correct  in  every  particular,  will  not  fail  to 
represent  the  observations  within  the  limits  of  their 
probable  errors.  The  results  of  these  computations 


C  0  M  FIXATION     OF     AN     O  11  B  I  T  .  165. 

give  the  elements  of  the  orbit  of  the  comet.  These 
elements  are  six  in  number — of  which  two  refer  to 
the  nature  and  magnitude  of  the  orbit,  three  to  its 
position  in  space,  and  one  to  the  position  of  the 
comet  in  its  orbit  at  a  given  time.  They  are,  there- 
fore, the  following :  1st.  The  perihelion  distance,  or 
distance  between  the  focus  and  vertex  of  the  curve, 
the  sun  being  placed  in  the  focus ;  2d.  The  eccen- 
tricity, which  in  the  case  of  the  parabola  becomes  a 
known  quantity ;  3d.  The  inclination  of  the  plane 
of  the  orbit  of  the  comet  to  the  plane  of  the  earth's 
orbit,  or  to  what  is  called  the  ecliptic;  4th.  The  posi- 
tion of  the  line  in  which  the  orbit  intersects  the 
ecliptic  ;  5th.  The  position  of  the  greater  axis  of  the 
orbit  in  space ;  and  6th.  The  time  at  which  the 
comet  was  in  its  perihelion,  or  its  angular  distance 
from  this  point  at  any  stated  epoch.  In  order  to  fix 
the  position  of  the  line  of  intersection  of  the  orbit 
and  ecliptic,  —  which  is  called  the  line  of  nodes, — 
and  also  the  position  of  the  greater  axis  of  the  orbit, 
it  is  necessary  to  adopt  some  standard  point  of 
departure,  to  which  all  angular  measurements  may 
be  referred.  The  point  used  by  astronomers  is  that 
in  which  the  sun,  in  its  apparent  annual  course, 
passes  from  the  southern  to  the  northern  hemisphere 
of  the  heavens,  and  is  termed  the  vernal  equinox. 
When  the  comet  is  in  the  line  of  nodes,  in  the  act 
of  passing  from  the  south  to  the  north  side  of  the 


166  TREATISE   ON   COMETS. 

ecliptic,  it  is  said  to  be  in  its  ascending  node,  and  the 
position  of  the  line  of  nodes  is  determined  by  the 
angular  distance  of  the  ascending  node  from  the 
vernal  equinox.  This  angular  distance  is  called  the 
longitude  of  the  node.  The  position  of  the  greater 
axis  of  the  orbit  is  determined  by  the  angular  dis- 
tance between  the  vernal  equinox  and  the  perihelion 
point,  projected  orthographically  on  the  ecliptic. 
This  distance  is  called  the  longitude  of  the  perihelion. 
Such  are  the  elements  required  for  a  complete 
knowledge  of  all  the  past  and  future  circumstances 
of  the  motion  of  a  heavenly  body  revolving  around 
the  sun,  in  obedience  to  the  law  of  universal  gravi- 
tation. We  have  already  remarked,  that  if  we  knew 
the  exact  conditions  under  which  the  comets  were 
first  put  in  motion,  we  could  assign  the  proper  values 
of  these  elements  at  once,  without  the  aid  of  obser- 
vations; but  since  these  conditions  are  unknown, 
we  are  compelled  to  reverse  the  process,  and  by 
means  of  the  effects  produced  to  seek  out  the  causes 
which  originally  operated.  To  accomplish  this, 
three  observed  positions  of  the  comet  are  required. 
These  observations  should  be  taken  at  intervals  of 
a;t  least  one  day,  and  should  be  referred  to  the  cen- 
tre of  the  earth.  These  three  observations  give  us 
the  direction  of  the  three  visual  rays  drawn  from 
the  earth  to  the  comet  at  the  times  of  the  observa- 
tions, and  in  the  prolongation  of  which  the  latter 


COMPUTATION   OF  AN   ORBIT.         167 

must  necessarily  be  found.  The  corresponding 
places  of  the  sun  are  known  either  by  observation, 
or  by  calculation  from  the  solar  tables.  It  remains 
then  to  find  a  parabola,  ellipse,  or  hyperbola,  having 
its  focus  at  the  centre  of  the  sun,  and  cutting  the 
visual  rays  in  points,  the  intersections  of  which  cor- 
respond to  the  number  of  days  between  the  obser- 
vations— the  motion  in  the  orbit  being  in  accordance 
with  Kepler's  laws.  In  order  to  effect  this,  it  is 
necessary,  in  the  first  place,  to  refer  the  positions  of 
the  comet  as  seen  from  the  earth,  to  those  in  which 
it  would  appear  if  seen  from  the  centre  of  the  sun ; 
or,  in  other  words,  to  transform  the  geocentric  places 
of  the  comet  into  heliocentric  positions.  This  is 
accomplished  by  computing  a  series  of  triangles, 
formed  by  joining  the  successive  places  of  the  earth 
and  comet  with  the  sun,  and  by  joining  the  places 
of  the  earth  with  those  of  the  comet.  The  solution 
of  these  triangles,  together  with  Lambert's  theorem, 
in  the  case  of  a  parabola,  furnish  the  means  of  pass- 
ing from  the  earth  to  the  sun.  As  soon  as  the  helio- 
centric places  are  found  corresponding  to  the  dates 
of  the  observations  respectively,  it  becomes  an  easy 
process  to  determine  the  position  of  the  orbit  in 
space,  since,  at  the  centre  of  the  sun,  the  real  and 
apparent  motions  of  the  comet  are  the  same.  The 
transformation  which  gives  the  heliocentric  longi- 
tude and  latitude  of  the  comet,  gives  at  the  same 


168  TREATISE   ON   COMETS. 

time  its  distance  from  the  centre  of  the  sun,  or  what 
is  called  its  radius-vector,  corresponding  to  the  three 
observations.  "We  shall  thus  have  three  points  of 
the  curve  given ;  and  by  finding  the  ellipse,  para- 
bola, or  hyperbola,  as  the  case  may  be,  which  shall 
pass  through  these  three  points,  a  segment  of  the 
orbit  will  be  given.  Now,  the  properties  of  these 
curves  have  been  so  completely  investigated,  that  if 
a  segment,  —  no  matter  how  small,  —  is  given,  the 
entire  curve  may  be  readily  traced  out.  This  ena- 
bles us  to  determine  the  perihelion  distance  and  the 
eccentricity  of  the  orbit,  in  the  case  of  the  ellipse 
and  hyperbola,  the  latter  being  already  known  in 
the  case  of  the  parabola.  The  time  of  passing  the 
perihelion  is  then  found,  by  means  of  Kepler's  laws, 
from  the  times  of  the  observations. 

Such  is  a  concise  explanation  of  the  method  of 
determining  the  elements  of  the  orbit  of  a  comet  from 
three  observations;  and,  although  the  process,  as 
here  stated,  may  seem  to  be  by  no  means  difficult, 
yet  it  should  be  understood  that  this  problem,  if 
properly  solved,  is  one  of  the  most  intricate  and 
laborious  of  all  the  problems  which  the  science  of 
astronomy  affords.  Moreover,  a  detailed  account 
of  the  operations  to  be  performed  in  making  the 
computations,  would  require  a  higher  degree  of 
mathematical  knowledge,  in  order  to  be  fully  com- 
prehended, than  is  possessed  by  those  for  whom 


EPHEMERIS   OF   A   COMET.  169 

these  explanations  are  intended;  and  what  has 
already  been  stated  will  be  sufficient  to  exhibit,  in 
an  intelligible  manner,  the  general  character  of  the 
problem,  and  also  of  its  solution. 

When  the  elements  of  the  orbit  of  a  comet  have 
been  determined,  it  becomes  an  easy  process  to 
predict  its  position  in  the  heavens  for  any  future 
date.  It  is  customary  among  astronomers,  as  soon 
as  the  orbit  is  known,  should  the  comet  continue 
visible,  to  compute  a  series  of  places,  for  each  suc- 
cessive day,  during  the  period  of  its  visibility. 
These  places  are  indicated  by  means  of  the  com- 
puted right  ascension  and  declination  of  the  comet — 
the  former  being  the  angular  distance  between  it 
and  the  vernal  equinox,  measured  on  the  ecliptic 
towards  the  east ;  and  the  latter,  its  angular  distance 
north  or  south  from  the  celestial  equator.  The  dis- 
tance of  the  comet  from  the  earth  and  sun,  respect- 
ively, is  also  determined  at  the  same  time.  The 
positions  of  the  comet  thus  arranged  constitute  an 
ephemeris ;  and,  in  case  the  elements  have  been 
accurately  determined,  all  the  various  changes  in 
the  brilliancy  of  the  nucleus  or  head  may  be  pre- 
dicted for  many  months  in  advance. 

In  computing  an  ephemeris  it  is  necessary  to  find 

the  position  of  the  comet  in  its  orbit  for  a  given 

epoch,  by  means  of  the  time  of  perihelion  passage, 

the  eccentricity  of  the  orbit,  and  the  perihelion  dis- 

15 


170  TREATISE   ON    COMETS. 

tance.  This  being  known,  the  heliocentric  place 
of  the  comet  is  readily  found ;  and  then,  by  an  easy 
transformation,  the  geocentric  right  ascension  and 
declination  may  be  determined.  In  practice,  how- 
ever, a  course  slightly  different  from  this  is  pursued. 
The  position  of  the  comet  in  its  orbit  is  determined 
as  here  explained ;  but  instead  of  finding  the  helio- 
centric place,  and  from  it  the  geocentric  place,  the 
position  of  the  comet  with  respect  to  three  imagi- 
nary planes,  at  right  angles  to  each  other,  and 
having  their  point  of  common  intersection  in  the 
centre  of  the  sun,  is  investigated.  These  planes 
are  called  co-ordinate  planes,  and  the  point  of  com- 
mon intersection  is  called  the  origin.  The  three 
lines  of  intersection  of  the  planes  are  called  co-ordi- 
nate axes,  and  the  distances  of  the  comet  from  the 
origin,  measured  in  the  direction  of  these  lines,  are 
called  its  rectangular  co-ordinates.  The  rectangular 
co-ordinates  of  the  earth  are  then  found  by  means 
of  the  solar  tables,  which,  being  subtracted  from 
those  of  the  comet,  give  the  rectangular  co-ordinates 
of  the  comet  referred  to  the  centre  of  the  earth. 
In  making  these  transformations,  the  plane  of  the 
equator  is  usually  adopted  as  the  fundamental  plane, 
and  is  supposed  to  coincide  with  one  of  the  co-ordi- 
nate planes.  The  ecliptic,  also,  may  be  adopted  as 
the  fundamental  plane ;  but,  in  this  case,  the  final 
.places  of  the  comet  would  be  its  latitude  and  Ion- 


EPHEMERIS   OF   A   COMET.  171 

gitude  hi  the  heavens,  instead  of  its  right  ascension 
and  declination.  When  the  geocentric  rectangular 
co-ordinates  have  thus  heen  computed,  they  may  be 
transformed  at  once  into  polar  co-ordinates,  thus 
giving  the  apparent  position  of  the  comet  on  the 
celestial  vault.  It  should  be  remarked,  however, 
that  there  are  certain  small  corrections  which  must 
be  added  to  these  results,  in  order  to  find  the  exact 
place  of  the  comet,  but  which  are  not  of  sufficient 
importance  to  be  noticed  in  this  connection.  The 
ephemeris  is  now  compared  with  the  actual  observa- 
tions of  the  comet;  and  should  any  discrepancies  be 
exhibited,  the  elements  are  corrected  so  as  to  reduce 
these  differences  within  the  limits  of  the  probable 
errors  of  the  observations.  In  this  way,  therefore, 
new  and  more  accurate  elements  are  obtained,  and 
an  exact  ephemeris  is  then  computed  precisely  as 
before. 

When  a  new  comet  has  been  discovered,  it  is  ob- 
served for  a  few  days,  and  as  soon  as  three  places 
are  obtained,  separated  by  an  interval  of  time  of 
sufficient  extent,  the  elements  are  computed  under 
the  supposition  that  the  orbit  is  a  parabola.  An 
ephemeris  is  then  computed  from  the  elements,  and 
compared  with  the  entire  series  of  observations,  in- 
cluding .an  interval  of  time  of  the  greatest  possible 
extent.  Should  it  be  found,  from  this  comparison, 
that  the  observations  are  not  satisfactorily  repre- 


172  TREATISE   ON   COMETS. 

sented,  the  elements  are  corrected  as  above  ex- 
plained; and  in  case  discordances  are  still  found  to 
exist,  it  becomes  necessary  to  abandon  the  hypo- 
thesis of  a  parabolic  orbit,  as  incompatible  with  the 
motion  of  the  comet.  An  ellipse  is  now  computed, 
and  compared  with  the  observations ;  and  if,  after 
having  been  finally  corrected,  it  is  still  impossible 
to  satisfy  the  entire  series  of  observations,  recourse 
is  had  to  the  hyperbola.  The  comparison  thus 
instituted  between  the  observations  and  the  com- 
puted places  resulting  from  the  hypothesis  of  a 
parabola,  an  ellipse,  or  an  hyperbola,  respectively, 
will  indicate  the  precise  character  of  the  orbit.  The 
final  effect  of  these  successive  hypotheses  on  the 
relation  of  the  comet  to  our  system,  may  be  readily 
conceived.  A  comet  which  moves  in  a  parabola 
cannot  make,  like  one  which  moves  in  an  ellipse,  a 
succession  of  revolutions  around  the  sun.  It  enters 
the  system  in  some  definite  direction,  but  as  pro- 
ceeding from  almost  an  indefinite  distance.  As 
soon  as  it  arrives  within  the  sensible  influence  of 
solar  gravitation,  the  effects  of  this  attraction  are 
manifested  in  the  curvilinear  form  which  its  path 
begins  to  assume,  and  which  gradually  increases  as 
its  distance  from  the  sun  decreases.  When  it- 
reaches  its  perihelion,  the  attractive  force,  and  con- 
sequently the  curvature  of  its  path,  have  attained 
their  maxima,  and  the  comet  has  now  also  acquired 


PERIODIC    COMETS.  173 

its  greatest  velocity  of  translation.  This  extreme 
velocity,  by  virtue  of  the  inertia  of  the  moving 
mass,  produces  a  centrifugal  force  which  counteracts 
gravitation ;  and  the  body,  having  passed  its  peri- 
helion, begins  to  retreat,  and  pursuing  a  similar 
path  to  that  in  which  it  previously  moved,  only  in  a 
reversed  order,  it  finally  passes  out  of  the  system 
in  nearly  a  straight  line,  and  parallel  to  that  in 
which  it  entered.  It  is  evident  from  this,  that  a 
comet  which  moves  in  such  an  orbit,  can  visit  the 
solar  system  but  once,  unless  it  may  happen  that  in 
passing  through  some  other  adjoining  system,  its 
course  is  so  changed  by  the  attractive  influences 
which  may  there  be  in  operation,  that  it  again 
retraces  its  way,  in  its  majestic  flight,  towards  our 
system,  but  in  a  direction  entirely  different  from 
that  in  which  it  first  entered. 

In  the  case  of  an  hyperbolic  orbit,  the  circum- 
stances of  the  motion  of  the  comet  with  respect  to 
the  sun  would  be  nearly  the  same  as  results  from 
parabolic  motion.  The  only  difference  would  be, 
that  in  the  parabola  the  comet  enters  the  system  in 
nearly  a  straight  line,  and  issues  from  it  in  a  straight 
line  parallel  to  this ;  while  in  the  hyperbola  these 
straight  lines  are  divergent,  and  not  parallel.  The 
comet,  in  either  of  these  cases,  would  not  have  a 
periodic  character.  It  might  be  said  to  be  analogous, 
in  this  respect,  to  one  of  those  occasional  meteors 
15* 


174  TREATISE   ON    COMETS. 

which  are  seen  to  shoot  across  the  firmament  never 
again  to  reappear.  The  body,  arriving  from  some 
distant  region,  and  coming,  as  would  appear,  for- 
tuitously within  the  solar  attraction,  is  drawn  from 
its  course  into  an  hyperbolic  or  parabolic  path,  which 
it  is  seen  to  pursue  ;  and  escapes  from  the  solar 
attraction,  issuing  from  the  system,  and,  for  aught 
we  know,  never  to  return.  The  phenomenon,  in 
each  case,  might  be  said  to  be  occasional,  and,  in  a 
certain  sense,  accidental,  and  the  body  could  not  be 
said  to  belong  properly  to  the  system.  So  far  as 
relates  to  the  comet  itself,  it  may  be  said  that  the 
phenomena  consists  in  a  change  of  the  direction  of 
its  course  through  the  universe,  influenced  by  the 
temporary  action  of  solar  gravitation  upon  it. 

Such  are  the  circumstances  under  which  a  great 
majority  of  the  comets  which  have  visited  our  sys- 
tem, have  been  found  to  move ;  and  they  may  on  this 
account  be  regarded  as  a  connecting  link  between 
the  various  systems  of  the  universe.  But  in  the  case 
presented  by  a  comet  moving  in  an  elliptic  orbit,  the 
relation  between  the  comet  and  our  system  is  much 
more  intimate,  and  the  interest  and  physical  import- 
ance of  the  body  transcendently  greater.  In  this 
case  the  comet  possesses  a  periodic  character,  making 
successive  revolutions  like  the  planets,  and  return- 
ing to  its  perihelion  at  regular  intervals  of  time. 
Unlike  those  comets  which  move  in  parabolas  or 


PERIODIC   COMETS.  175 

hyperbolas,  it  would  not  be  an  occasional  visitor  to 
our  system,  connected  with  it  by  no  permanent  re- 
lation, and  subject  to  solar  gravitation  only  acci- 
dentally and  temporarily.  On  the  contrary,  it  is  to 
be  regarded  as  being  as  permanent,  if  not  as  strictly 
regular,  a  member  of  the  system,  as  any  of  the 
planets,  although  invested  with  an  extremely  differ- 
ent physical  character.  This  being  the  most  plausi- 
ble view  of  the  cometary  motions  in  general,  and 
one  which  would  seem  to  have  impressed,  at  once, 
every  one  who  has  studied  the  phenomena  of  these 
wonderful  bodies,  it  may  perhaps  seem  strange  that 
no  attempt  was  made  until  recently  to  determine 
the  orbits  of  the  comets  under  the  direct  supposition 
of  elliptic  motion.  But  it  should  be  remembered 
that  those  which  are  known  to  be  periodic  move  in 
ellipses  of  great  eccentricity ;  and  since  they  are  ob- 
served only  when  near  their  perihelion,  it  becomes 
almost  impossible  to  decide  in  reference  to  the 
nature  of  the  orbit.  As  already  remarked,  the  ob- 
servations are  not  absolutely  exact ;  and  making  due 
allowance  for  the  probable  errors  of  observation, 
even  when  the  comet  has  been  visible  for  a  long 
period  of  time,  it  is  generally  found  that  its  visible 
path  may  be  represented  with  almost  equal  fidelity 
by  either  of  the  three  curves  in  which  these  bodies 
move.  In  all  such  cases,  it  is  impossible  to  infer, 
from  the  observations  alone,  whether  the  comet 


176  TREATISE   ON   COMETS. 

belongs  to  the  class  of  hyperbolic  or  parabolic 
bodies,  which  have  no  periodic  character,  or  to  the 
elliptic,  which  has. 

When  the  period  of  revolution  does  not  exceed 
100  years,  the  periodic  time  may  be  found  within 
a  year  of  the  true  time  from  the  observations  made 
during  two  or  three  months.  In  this  case  the  ellip- 
ticity  is  so  marked  that  the  observations  will  not 
fail  to  indicate  it.  "When  the  period  is  only  about 
five  or  fifteen  years,  the  observations  of  three  months, 
provided  they  are  numerous,  will  give  the  orbit  so 
approximately  that  the  next  appearance  of  the  comet 
may  be  predicted  within  a  few  days  of  the  observed 
time.  The  method  of  determining  the  elements  of 
the  orbits  of  the  heavenly  bodies  has  been  carried 
to  so  high  a  degree  of  perfection,  that  by  combining 
many  observations  made  at  different  places  nearly 
simultaneously,  or  within  a  few  days  of  each  other, 
thus  forming  what  are  called  normal  places  of  the 
comet,  almost  the  exact  form  and  magnitude  of  the 
orbit  can  be  found  directly.  But  although,  in  the 
case  of  a  periodic  comet,  the  ellipticity  is  plainly 
exhibited,  and  its  value  very  closely  approximated, 
yet  the  unavoidable  errors  of  observation  are  still 
sufficient  to  render  the  period  of  revolution  to  a 
limited  extent  uncertain.  Sometimes  this  uncer- 
tainty may  amount  to  several  weeks,  and  again  it 
may  not  exceed  a  few  days  or  perhaps  a  few  hours 


PKHIODIC   COMETS.  177 

in  the  time  of  the  return  of  the  comet  to  its  perihe- 
lion. It  depends  necessarily  on  the  character  and 
number  of  the  observations  employed,  and  also  upon 
the  interval  of  time  during  which  the  comet  was 
accurately  observed,  or  in  a  position  favorable  for 
good  observations. 

It  often  happens  that  the  character  of  periodicity 
itself,  which  of  course  belongs  only  to  elliptic  orbits, 
supplies  the  means  of  surmounting  the  difficulties 
just  enumerated  as  affecting  the  exact  determination 
of  the  periodic  time  of  a  comet,  from  the  observa- 
tions made  during  a  brief  interval.  If  any  observed 
comet  has  an  elliptic  orbit,  it  must  return  again  to 
its  perihelion  after  having  completed  one  revolution, 
and  it  must  also  have  been  visible  on  former  returns 
to  that  position.  It  is  evident,  therefore,  that  not- 
only  ought  the  comet  to  be  expected  to  reappear  in 
future  at  regular  intervals,  but  that  its  previous  re- 
turns to  its  perihelion  would  be  found,  in  case  it  had 
been  observed,  by  searching  among  the  recorded 
appearances  of  such  objects  for  any,  the  dates  of 
whose  appearances  might  correspond  with  the  sup- 
posed period,  and  whose  apparent  motions  might 
indicate  a  real  motion  in  an  orbit,  identical,  or  nearly 
so,  with  that  of  the  comet  which  may  be  under  con- 
sideration. ~Now  it  is  known  that,  in  most  cases, 
the  elements  of  the  orbit  undergo  comparatively 
slight  changes  during  one,  or  even  two  or  three  suc- 

M 


178  T  K  E  A  T  I  S  E     ON     C  0  M  E  T  S  . 

cessive  revolutions.  If  the  force  of  gravity  of  the 
sun  operated  alone,  and  the  comet  was  not  subjected 
to  the  attractions  of  the  various  planets  of  the  sys- 
tem, the  elements  would  continue  invariable.  The 
comet  would  reappear  after  each  successive  revolu- 
tion at  exactly  the  same  point  as  seen  from  the  sun  ; 
would  follow,  while  visible,  exactly  the  same  arc  of 
its  orbit ;  would  move  in  the  same  plane,  inclined  at 
the  same  angle  to  the  ecliptic,  the  nodes  retaining 
the  same  places  ;  and  would  arrive  at  its  perihelion 
at  exactly  the  same  point  and  after  exactly  equal 
intervals  of  time.  The  disturbing  action  of  the 
planets  is  usually  small,  especially  when  the  inclina- 
tion of  the  plane  of  the  orbit  of  the  comet  to  the 
plane  of  the  ecliptic  is  considerable.  In  general, 
this  disturbance,  which  is  called  the  planetary  per- 
turbations, will  not  change  any  of  the  elements  more 
than  one  or  two  degrees ;  while  in  many  instances 
the  variations  in  the  position  of  the  orbit  in  space 
with  respect  to  the  equinox  and  ecliptic — these  being 
the  elements  which  are  most  affected — will  not 
exceed  half  a  degree.  It  may  happen,  however,— 
the  orbit  of  the  comet  being  very  eccentric,  and  on 
that  account  nearly  intersecting  the  paths  of  several 
planets — that  the  disturbing  action  of  these  planets 
may,  in  the  aggregate,  entirely,  or  nearly  overcome 
the  principal  effect  of  the  predominant  force  of  the 
sun ;  and  the  orbit  of  the  comet  at  two  different  re- 


PERIODIC   COMETS.  179 

turns  will  be  so  completely  changed  that  it  would 
be  impossible,  from  a  mere  inspection  of  the  elements 
corresponding  to  each  appearance,  to  identify  it. 
While,  therefore,  we  may  be  prepared  for  the  possi- 
bility, and  even  the  probability,  that  the  same 
periodic  comet  on  the  occasion  of  its  successive  re- 
appearances, may,  in  passing  to  and  from  its  peri- 
helion, follow  a  path  differing  to  some  extent  from 
that  which  it  had  followed  at  previous  returns ;  yet, 
as  we  have  already  remarked,  such  differences  can- 
not, except  in  rare  and  exceptional  cases,  be  very 
considerable.  For  the  same  reason  the  intervals 
between  its  successive  periods,  though  they  may 
differ,  cannot  be  subject  to  any  great  variation. 

The  elements  of  the  orbits  of  all  the  comets  which 
have  appeared  during  the  last  century,  and  also  of 
many  comets  which  had  previously  appeared,  have 
been  computed  and  recorded.  As  soon,  therefore, 
as  an  unexpected  comet  has  made  its  appearance, 
and  the  elements  of  its  orbit  have  been  determined, 
an  examination  of  these  recorded  elements  is  made, 
in  order  to  detect,  if  possible,  any  which  have  a 
sufficient  resemblance  to  those  of  the  comet  in  ques- 
tion, to  warrant  the  supposition  of  their  identity. 
Should  it  be  found,  on  making  such  a  comparison, 
that  there  are  no  elements  on  record  which  satisfy 
the  required  conditions  of  identity,  we  may  conclude 
that  the  comet  is  a  new  one.  and  must  have  recourse 


180  TREATISE   ON    COMETS. 

to  actual  observations  for  the  determination  of  the 
precise  character  of  its  motion.  The  approximate 
elements  already  known  will  enable  us  to  conjecture 
at  least  the  probable  influence  of  the  principal 
planets,  and  we  are  thus  enabled  to  make  due  allow- 
ance, in  the  comparison,  for  any  discrepancies  in  the 
elements  which  might  result  from  this  cause.  In 
the  case  of  a  parabola,  there  are  five  elements  to  be 
compared ;  and  since,  considering  a  limited  number 
of  comets,  the  chances  are  so  small  that  a  near  re- 
semblance of  all  the  elements  may  be  accidental,  we 
may  conclude  without  hesitation,  in  all  cases  where 
such  a  resemblance  is  detected,  that  the  comets  are 
identical,  and  that  whatever  differences  are  found 
to  exist,  must  be  due  to  the  attraction  of  the  planets 
and  to  the  unavoidable  errors  of  the  observations. 

The  identity  of  a  comet  thus  determined  does  not 
always  furnish  the  period  of  its  revolution.  It  is 
not  known  whether  the  comet  has  made  but  one 
revolution  during  the  interval  which  has  elapsed,  or 
whether  it  may  not  have  returned  several  times  un- 
perceived.  This  question  must  be  decided  by  actual 
computations  from  the  observations.  If  it  is  found 
that,  by  assuming  an  eccentricity  less  than  that 
which  would  result  from  the  supposition  of  a  single 
revolution,  the  observations  may  be  more  completely 
represented,  it  will  be  evident  that  the  comet  has 
made  more  than  one  revolution  between  the  epochs 


PERIODIC   COMETS.  181 

of  the  two  sets  of  elements.  By  successive  approxi- 
mations, conducted  in  the  same  manner,  or  perhaps 
by  direct  computation  of  elliptic  elements  from  the 
observations,  this  question  may  be  definitely  settled, 
and  the  periodic  time  may  thus  be  accurately  deter- 
mined. 

Nor  is  the  fact  that  no  identity  can  be  found  by  a 
comparison  of  the  elements  of  a  comet  which  may 
be  under  consideration,  with  those  which  have  been 
recorded,  in  itself  sufficient  evidence  of  its  not 
having  a  fixed  period  of  revolution.  It  may  have 
returned  always  unperceived,  or  it  may  have  been 
seen  but  not  observed  sufficiently  for  the  determi- 
nation of  its  orbit.  Prior  to  the  time  of  Kepler  and 
Galileo,  these  bodies  were  often  visible  for  a  long 
period  of  time,  but  were  not  observed,  except  as 
objects  of  curiosity,  or  as  portending  the  greatest 
imaginable  disasters.  Indeed,  we  find  that  historians 
have  mentioned,  and  even  described,  their  appear- 
ances, and  in  some  cases  have  indicated  the  principal 
constellations  through  which  such  bodies  passed, 
although  no  observations  of  their  apparent  places 
have  been  transmitted,  by  which  any  close  approxi- 
mation to  their  actual  paths  could  be  made.  If, 
therefore,  the  elements  of  a  comet  are  known  so 
approximately  as  to  give  its  period  of  revolution 
within  limits  of  a  few  years,  and  it  is  found  that 
comets  have  been  thus  described  at  epochs  which 
16 


182  TREATISE   ON    COMETS. 

might  correspond  with,  its  previous  appearances,  the 
identity  is  possible.  In  such  cases  it  is  only  neces- 
sary to  reduce  the  modern  elements  to  the  ancient 
epoch,  and  by  computing  the  path  of  the  comet, 
find  whether  it  would  have  presented  the  phenomena, 
both  in  respect  to  brilliancy  and  apparent  motion, 
which  were  then  observed.  Should  they  be  found 
to  agree,  it  is  certain  that  the  comets  are  identical. 
The  resemblance  in  magnitude,  form,  and  splen- 
dor, which  may  be  found  to  exist  between  comets 
which  have  appeared  at  different  epochs,  does  not 
afford  any  reasonable  basis  for  supposing  them  to 
have  been  successive  appearances  of  the  same  comet. 
In  modern  times  more  reliable  data  are  furnished 
for  the  solution  of  this  problem ;  while  in  more 
ancient  times,  the  only  circumstance  generally  re- 
corded was  the  appearance  of  the  object,  accom- 
panied, in  many  instances,  with  details  bearing 
evident  marks  of  exaggeration.  It  is  only,  there- 

• 

fore,  in  cases  where  the  computations  of  the  elements, 
from  modern  observations,  indicate  unequivocally 
the  elliptic  form  of  the  orbit,  and  its  approximate 
periodic  time  ;  or,  where  a  distinct  and  close  resem- 
blance between  two  different  systems  of  elements 
for  comets  at  two  different  epochs,  respectively,  is 
found  to  exist ;  that  the  precise  period  of  revolution 
of  a  comet  can  be  made  known,  except  in  the  case 
already  referred  to,  in  which  the  period  does  not 


PERIODIC    COMETS.  183 

exceed  fifteen  years.  It  may  thus  be  perceived  how 
astronomers  are  enabled  to  make  such  wonderful 
predictions  in  regard  to  the  future  movements  of 
some  of  these  objects,  as  have  been  made  from  time 
to  time.  In  no  other  science  can  be  found  so  mar- 
vellous a  series  of  phenomena  foretold.  The  interval 
between  the  prediction  and  its  fulfillment  has  some- 
times exceeded,  and  will  often  exceed,  the  limits  of 
human  life ;  and  one  generation  bequeathes  its  pre- 
dictions to  another,  which  is  to  be  filled  with  aston- 
ishment and  admiration  at  witnessing  their  literal 
accomplishment. 

We  have  thus  explained  in  a  concise,  yet  suffi- 
ciently extended  manner,  the  process  by  which 
astronomers  are  enabled  to  predict  the  past  and 
future  movements  of  a  comet,  from  observations 
made  only  during  a  brief  period  in  which  it  is 
visible.  In  the  explanations  here  given,  it  was,  of 
course,  impossible  to  enter  into  a  detailed  account 
of  all  the  various  methods  which  may  be  employed 
in  the  solution  of  this  problem.  We  have  given 
that  which  is  most  simple  and  readily  understood, 
and  which  is  sufficient  to  demonstrate  to  those  unac- 
quainted with  the  abstruse  and  intricate  mathe- 
matical investigations  which  are  effected  by  the 
professional  astronomer  in  making  these  calcula- 
tions, the  theory  of  the  solution,  thus  showing  con- 
clusively that  the  final  results  arrived  at,  though 


184  TREATISE   ON    COMETS. 

sometimes  startling,  are  of  the  most  positive 
character. 

In  the  preceding  chapter  we  gave  a  description 
of  some  of  the  most  remarkable  comets  which  have 
appeared  —  reserving,  however,  until  now,  a  de- 
scription of  the  phenomena  connected  with  the 
periodic  comets,  or  at  least  those  which  have  either 
been  identified  at  more  than  one  appearance,  or  are 
known  to  have  periods  of  revolution  not  exceeding 
one  or  two  hundred  years.  We  shall  therefore  pro- 
ceed at  once  to  give  an  account  of  this  latter  class 
of  comets,  in  the  order  in  which  their  periodic  char- 
acter has  been  discovered. 

It  might,  indeed,  be  expected  that  comets  moving 
in  elliptic  orbits  of  small  dimensions,  and  conse- 
quently having  short  periods,  would  have  been  the 
first  in  which  the  character  of  periodicity  would  be 
discovered.  The  comparative  frequency  of  their 
returns  to  their  perihelia,  and  the  consequent  possi- 
bility of  verifying  the  fact  of  their  periodic  charac- 
ter, together  with  the  distinctly  elliptic  form  of  their 
orbits,  which  would  be  supposed  to  be  made  evident 
by  computation,  would  afford  a  firm  basis  for  such 
an  expectation.  But  it  is  found  in  this  case,  as  has 
happened  in  so  many  others  in  the  progress  of  phy- 
sical science,  that  the  actual  results  of  observation 
and  research  have  been  directly  contrary  to  such  an 
anticipation ;  the  most  remarkable  case  of  a  comet 


PERIODIC   COMETS.  185 

of  large  orbit,  long  period,  and  consequently  rare 
returns,  being  the  first,  and  those  of  small  orbits, 
short  periods,  and  frequent  returns,  the  last,  gene- 
rally speaking,  whose  periodicity  has  been  disco- 
vered. 

Soon  after  the  discovery  of  the  law  of  gravitation 
by  Newton,  it  was  found  to  confirm  completely  the 
laws  of  Kepler,  and  also  to  be  constant  and  univer- 
sal in  every  situation  in  which  it  could  be  traced, 
whether  on  the  earth  or  in  the  solar  system.  This 
fact  gave  to  the  physical  sciences  generally,  and  to 
that  of  astronomy  in  particular,  a  generalization  and 
harmony  which  had  not  previously  been  known. 
The  general  law  of  the  motions  of  the  heavenly 
bodies  through  space,  in  connection  with  the  obser- 
vations of  the  great  comet  of  1680,  led  Newton  to 
conclude  that  the  orbits  of  the  comets  must,  like 
those  of  the  planets,  be  ellipses  having  the  sun  in 
one  focus,  but  far  more  eccentric ;  and  having  their 
aphelia,  or  greatest  distances  from  the  sun,  far  re- 
mote in  the  regions  of  space.  He  adds,  however, 
that  he  leaves  it  to  others  to  determine  the  greater 
axes  of  their  orbits  and  their  periods  of  revolution, 
by  comparing  comets  which  return  after  long  inter- 
vals of  time,  with  the  same  orbits.  The  idea  thus 
advanced  by  Newton,  extraordinary  as  it  may  ap- 
pear, was  soon  to  be  strictly  realized.  Twenty  years 
later,  Halley  collected  all  the  observations  of  comets 
16* 


186  TREATISE   ON    COMETS. 

which  he  could  procure ;  and,  by  means  of  the 
method  invented  by  Newton,  he  determined  with 
great  difficulty  the  elements  of  the  orbits  of  twenty- 
four,  which,  out  of  425  of  these  bodies  recorded 
prior  to  the  beginning  of  the  eighteenth  century, 
were  all  that  had  been  observed  with  sufficient 
accuracy  to  admit  of  the  determination  of  their 
orbits  by  the  method  which  he  employed. 

On  comparing  the  elements  thus  obtained,  Halley 
found  that  the  orbits  of  the  comets  of  1531,  1607, 
and  1682,  were  nearly  the  same.  He  also  found 
that  one  which  had  appeared  in  1661,  followed 
nearly  the  same  path  with  one  which  was  visible  in 
1532.  He  was,  therefore,  led  to  conclude  that  the 
former  comets  were  reappearances  of  the  same 
comet,  which  would  thus  have  a  period  of  revolution 
of  about  75  or  76  years,  and  that  the  latter  had  a 
period  of  129  years.  It  may  perhaps  be  proper  to 
add  that  the  latter  conjecture  has  failed  to  be  real- 
ized, while  the  former  has  been  found  to  be  literally 
true.  His  belief  of  the  identity  of  the  comets  of 
1607  and  1682,  was  still  further  confirmed  by  records 
of  more  ancient  comets,  among  which  he  found 
three  nearly  corresponding  in  their  periods  with  the 
former.  These  were  the  comets  of  1305,  1378,  and 
1456.  He  now  declared  his  opinion  that  the  same 
comet  had  appeared  at  these  six  epochs ;  and  that, 
since  its  period  of  revolution  was  a  little  more  than 


HALLEY'S   COMET.  187 

seventy-five  years,  it  might  be  expected  to  return  in 
the  year  1758.  The  following  table  shows  the  re- 
semblance of  the  periods  which  led  to  this  important 
announcement : 

Perihelion  Passage.  Interral. 

November    9,  1378,  --  - 

'          '  •     •     •     77  years,    7    months. 

June              9,  1456,  75  "        9*        " 

August       26,  1531,  76  „        2*        « 

October       27,  1607,  ?4  u      1(U       tt 
September  15,  1682, 

Here  was  exhibited  an  inequality  in  the  periodic 
time  amounting  to  more  than  two  years — a  circum- 
stance which,  in  connection  with  the  fact  that  there 
was  found  to  be  a  variation  in  the  inclination  of  the 
orbit  of  the  comet  to  the  plane  of  the  ecliptic,  might 
seem  to  cast  a  doubt  on  the  conclusion  at  which 
Halley  had  arrived.  It  was  known,  however,  that 
the  comet  might  have  been  sensibly  disturbed  in  its 
orbit  by  the  attractions  of  Jupiter  and  Saturn  at 
its  successive  returns  ;  and  that  this  would  perhaps 
account  for  the  discrepancies  in  its  periodic  time, 
which  would  otherwise  appear  to  exist.  The  attrac- 
tion of  Jupiter  on  Saturn  was  known  to  affect  the 
velocity  of  the  latter  planet;  sometimes  retarding 
and  sometimes  accelerating  it,  according  to  their 
relative  position,  so  as  to  affect  its  period  to  the 
extent  of  thirteen  days.  The  influence  of  Saturn 
on  Jupiter  was  also  known  to  be  of  precisely  a  sim- 
ilar character,  although  its  effect  was  not  more  than 


f 

188  THEATISE   ON    COMETS. 

half  as  great.  Under  such  circumstances,  Halley 
did  not  hesitate  to  declare  that,  if  such  heavy  bodies 
as  the  planets  could  be  disturbed  in  their  motions 
by  their  mutual  attractions,  a  comet,  —  which  he 
considered  a  mass  of  vapor,  —  would  be  still  more 
disturbed  in  passing  near  such  large  and  solid  bodies, 
especially  if  far  removed  from  the  influence  of  the 
sun.  He  concluded,  also,  that  since  the  motion  of 
the  comet  in  its  orbit  is  so  rapid,  a  very  small  in- 
crease of  velocity  from  any  disturbing  cause  would 
change  the  form  of  its  ellipse. 

For  these  and  similar  reasons,  he  did  not  hesitate 
to  attribute  the  inequality  observed  in  the  interval 
between  its  successive  returns,  and  also  the  variation 
in  the  inclination  of  its  orbit,  to  the  attractive  influ- 
ence of  planets.  Moreover,  he  noticed  that  in  the 
interval  between  1607  and  1682,  the  comet  passed 
so  near  Jupiter,  that  his  attraction  must  have  aug- 
mented its  velocity,  and  consequently  shortened  its 
period.  In  the  same  manner  he  found  that,  after 
1682,  its  period  would  be  again  increased  by  the 
influence  of  Jupiter  and  Saturn ;  and  he  therefore 
predicted,  finally,  that  the  comet  would  reappear 
toward  the  latter  part  of  the  year  1758,  or  about 
the  beginning  of  1759.  It  was  certainly  a  bold 
analogy,  in  those  days,  to  attribute  a  difference  of 
more  than  a  year,  in  the  motion  of  a  comet,  to  the 
same  cause  that  occasioned  a  change  of  only  a  few 


II  A  L  L  E  Y  '  s    COMET.  189 

days  in  the  motions  of  the  planets ;  and  this  confi- 
dent prediction  by  Halley  may  therefore  be  regarded 
as  one  of  the  most  remarkable  events  in  the  history 
of  astronomy.  But,  great  as  was  this  achievement, 
the  imperfect  state  of  mathematical  science  rendered 
it  impossible  for  Halley  to  exhibit  to  the  world  an 
absolute  demonstration  of  the  event  which  he  fore- 
told. It  was,  therefore,  only  possible  for  him  to 
announce  these  felicitous  conceptions  of  a  sagacious 
mind  as  mere  intuitive  perceptions,  which  must  be 
received  as  uncertain  by  the  world,  —  however 
strongly  he  himself  may  have  been  impressed  with 
them,  —  until  they  could  be  verified  by  the  process 
of  a  rigorous  analysis. 

Such  was  the  prediction  of  Halley — a  prediction 
which,  considering  the  state  of  astronomical  science 
at  the  time  it  was  made,  must  certainly  be  regarded 
as  one  of  the  most  extraordinary  and  wonderful 
which  has  ever  been  made.  The  distinguished  phi- 
losopher, sensible  of  the  fact  that  he  could  not  live 
to  witness  the  verification  of  his  prophetic  announce- 
ment, patriotically  expressed  the  hope  that,  should 
the  comet  return  in  accordance  with  his  prediction, 
posterity  would  do  him  the  justice  to  acknowledge 
that  it  was  announced  by  an  English  astronomer. 

Before  the  comet  had  accomplished  another  revo- 
lution, at  the  completion  of  which  Halley's  predic- 
tion was  to  be  fully  realized,  mathematical  science 


190  TREATISE   ON    COMETS. 

had  advanced  rapidly  to  a  very  high  degree  of  per- 
fection, and  men  of  sublime  genius  had  arisen,  who 
were  able  to  follow  out  Newton's  theory  of  gravita- 
tion to  its  legitimate  results  and  consequences.  Of 
the  problems  thus  presented  for  investigation  there 
was  one  of  peculiar  importance,  and  yet  of  the  most 
difficult  character.  One  of  the  first  and  simplest 
results  of  the  theory  of  gravitation  was,  that  if  a 
single  planet  revolved  around  the  sun  in  free  space 
—  its  mass  being  supposed  to  be  insignificant  as  com- 
pared with  that  of  the  sun  —  it  must  revolve  in  an 
ellipse,  the  focus  of  which  must  be  occupied  by  the 
centre  of  the  sun.  But  if  we  now  suppose  a  second 
planet  to  be  admitted  into  the  system,  then  the 
ellipticity  of  their  orbits  can  be  preserved  only  on 
the  supposition  that  the  two  planets  have  no  attrac- 
tion for  each  other,  and  that  no  physical  force  is  in 
operation  except  the  solar  attraction.  This  will 
necessarily  require  that  their  masses  shall  be  so  small, 
compared  with  that  of  the  sun,  as  to  be  entirely 
neglected.  The  law  of  universal  gravitation,  how- 
ever, is  based  on  the  principle  that  every  particle  of 
matter,  every  body  in  nature,  must  be  mutually 
attracted.  This  may  seem  to  indicate  that  the  elliptic 
character  of  the  orbit  would  be  effaced ;  but  it  should 
be  remembered,  that  in  the  cases  of  the  planets  of 
the  solar  system,  their  masses  are  so  small  compared 
with  that  of  the  sun,  that  for  any  limited  time  their 


H ALLEY'S    COMET.  191 

orbits  may  be  regarded  as  purely  elliptic.  But  in 
course  of  time  their  orbits  must  be  slightly  changed, 
depending  on  their  relative  masses,  those  whose 
masses  are  least  being  most  sensibly  affected.  These 
slight  deviations  are  called  the  perturbations  of  the 
planets  and  comets,  or  the  disturbances  due  to  their 
reciprocal  attraction.  The  problem  thus  presented 
is  known  as  the  problem  of  the  three  bodies,  and  its 
extension  embraces  the  effects  of  the  mutual  gravi- 
tation of  all  the  planets  of  the  system  upon  each 
other.  We  must,  however,  defer,  until  a  subsequent 
connection,  a  complete  explanation  of  the  method 
of  solving  this  problem,  and  of  its  more  extended 
applications  ;  stating  here  only  what  may  be  required 
in  order  to  understand  the  manner  in  which  the 
exact  time  of  the  return  of  a  comet  may  be  predicted. 
The  problem  of  the  three  bodies,  involving  the 
sun,  earth,  and  moon,  had  already  engaged  the 
attention  of  Euler,  D'Alembert,  and  Clairaut;  and 
from  the  comparative  smallness  of  the  third  body 
considered,  namely,  the  moon,  the  solution  was 
effected  with  considerable  facility.  But  in  order  to 
apply  the  problem  to  the  case  of  the  sun,  a  planet, 
and  a  comet,  it  was  necessary  in  the  first  place  to 
investigate  the  analytical  formulae  which  would 
represent  the  respective  motions  of  the  planet  and 
comet,  before  the  actual  place  of  the  latter,  at  any 
given  time,  could  be  determined,  supposing  the  ele- 


192  TREATISE   ON   COMETS. 

ments  of  the  orbit  at  a  fixed  epoch  to  be  known. 
This  was  attempted  by  Clairaut?  one  of  the  most 
profound  geometers  then  living ;  and  he  endeavored 
to  determine  the  path  of  the  comet  of  1682,  when 
attracted  by  the  sun  and  disturbed  by  a  planet.  In 
this  complicated  problem,  the  disturbing  action  of 
one  planet  only  can  be  estimated  at  a  time,  and  on 
this  account  it  becomes  necessary  to  repeat  the 
numerical  computations  for  each  disturbing  body. 
Moreover,  the  disturbing  action  of  a  planet  can  only 
be  computed  for  an  extremely  small  portion  of  the 
orbit  at  a  time,  and  the  sum  of  these  is  the  whole 
effect. 

It  was  enough  for  Clairaut  to  investigate  the  com- 
plicated formulae  by  which  these  computations  were 
to  be  accomplished.  He  was  indeed  eminently 
qualified  to  conduct  such  an  investigation,  and  the 
manner  in  which  it  was  accomplished  may  be  wit- 
nessed by  the  accuracy  of  the  final  results.  When 
the  analytical  part  of  the  problem  had  been  solved, 
the  laborious  numerical  computations  were  under- 
taken by  Lalande,  an  accomplished  practical  as- 
tronomer. The  fidelity  with  which  he  performed 
his  part  will  soon  be  made  evident.  In  this  pro- 
digious labor  he  was  assisted  by  Madame  Lepaute, 
an  accomplished  lady  of  Paris,  whose  name  —  al- 
though, for  some  unaccountable  reason,  wholly 
omitted  in  Clairaut' s  Memoir  giving  an  account  of 


HALLEY'S   COMET.  193 

the  computations  —  is  thus  deservedly  registered  in 
the  annals  of  astronomical  science. 

When  we  consider  that  the  period  of  the  comet  is 
about  seventy-five  years,  and  that  for  two  successive 
periods  it  was  necessary  that  every  portion  of  its 
orbit  should  he  calculated  separately,  as  above  ex- 
plained, a  general  idea  of  the  enormous  labor  per- 
formed by  Lalande  and  Madame  Lepaute  may  be 
formed.  They  computed  from  morning  till  night, 
and  even  late  in  the  evening,  without  intermission, 
for  a  period  of  six  months ;  having  computed  the 
distance  of  each  of  the  two  planets,  Jupiter  and 
Saturn,  from  the  comet,  and  their  attraction  upon 
that  body,  separately  for  each  degree  of  the  orbit, 
during  a  period  of  150  years.  The  result  was,  that 
in  consequence  of  the  attractive  influence  of  these 
two  planets,  the  period  of  revolution  of  the  comet 
would  be  lengthened  nearly  two  years.  It  was  found 
that  it  would  be  delayed  518  days  by  the  action  of 
Jupiter,  and  100  days  by  that  of  Saturn,  so  that  it 
would  arrive  at  its  perihelion  on  the  18th  of  April, 
1759.  It  was  afterwards  found,  on  a  revision  of  the 
calculations,  which  had  been,  toward  the  latter  part, 
performed  too  rapidly — in  some  instances  neglecting 
quantities  which  were  by  no  means  inconsiderable 
—  from  fear  of  being  anticipated  by  the  arrival  of 
the  comet,  that  the  perihelion  passage  would  take 
place  on  the  4th  of  April,  1759. 
17  N 


194  TREATISE   ON    COMETS. 

The  Memoir  which  contained  an  elaborate  account 
of  these  investigations  was  presented  to  the  French 
Academy  on  the  14th  of  November,  1758,  and  a 
supplementary  one,  containing  an  account  of  the 
revision  of  the  latter  part  of  the  numerical  com- 
putations, was  presented  a  few  weeks  later.  In  this 
Memoir  Clairaut  explained  the  unfavorable  circum- 
stances under  which  the  computations  were  made. 
He  had  no  observations  on  which  to  base  the  ele- 
ments of  the  orbit  but  those  of  Apian,  which  were 
far  from  being  accurate,  having  been  made  at  a  time 
when  little  attention  was  paid  to  comets.  It  is  pro- 
bable, however,  that  if  Apian  had  been  sensible  of 
their  future  importance,  he  would  have  observed 
with  much  greater  accuracy.  The  mass  of  Saturn 
was  unknown,  and  Clairaut  was  compelled  to  adopt 
a  value  which  has  since  been  found  to  be  erroneous. 
He  also  neglected  the  disturbing  action  of  the  earth, 
which  was  not  altogether  inconsiderable,  since  the 
comet  had  passed  near  it  in  1682,  and  could  not  take 
into  account  the  action  of  Uranus  and  Neptune, 
which  were  then  undiscovered.  The  effects  of  the 
action  of  Mercury,  Yenus,  and  Mars,  being  insig- 
nificant, were  not  computed. 

It  is  not  strange,  therefore,  that  Clairaut  admitted 
that  his  results  might  be  liable  to  a  small  error, 
enough,  perhaps,  to  leave  an  uncertainty  of  one 
month  in  the  date  of  the  return  of  the  comet  to  its 


H  A  L  L  K  Y  '  s   COMET.  195 

perihelion.  He  stated,  also,  that  there  might  be  very 
many  circumstances  which,  independent  of  any 
error  either  in  the  methods  or  process  of  calculation, 
might  cause  the  event  to  deviate  more  or  less  from 
its  predicted  occurrence.  He  was  well  aware  that  a 
body  which  passes  to  such  distant  regions  of  space, 
and  which  is  invisible  for  so  long  a  time,  may  be 
subjected  to  forces  wholly  unknown ;  such,  for  ex- 
ample, as  the  action  of  other  comets,  or  even  of  some 
planet  so  distant  from  the  sun  that  it  has  not  yet 
been  seen  —  a  fact  which  has  since  been  established 
by  the  discovery  of  the  planets  Uranus  and  Neptune. 
Again,  Newton  and  many  of  his  followers  believed  in 
the  existence  of  an  ethereal  fluid  throughout  all  space, 
although  there  was  no  proof  of  it  at  that  time,  and 
conjectured  that  it  would  accelerate  the  motions  of 
the  heavenly  bodies  without  altering  the  position  of 
their  orbits.  No  one,  however,  had  attempted  to 
estimate  its  effects  till  Clairaut  endeavored  to  find 
what  influence  it  would  have  on  the  motions  of 
Halley's  comet, — which  was  the  name  assigned  to 
the  comet  of  1682,  in  honor  of  the  discoverer  of  its 
periodic  time  —  and  he  found  that  the  acceleration 
would  not  amount  to  more  than  seven  and  a  half 
minutes. 

Such  were  the  circumstances  attending  the  pre- 
diction of  the  exact  time  of  the  comet's  return  by 
Clairaut;  and  so  positively  were  his  results  an- 


196  TKEATISE   ON   COMETS. 

nounced,  and  so  firmly  was  he  convinced  of  their 
general  accuracy,  that  he  felt  certain  that  the  comet 
would  first  be  visible  about  the  end  of  1758,  or  early 
in  January,  1759.  There  was,  however,  one  circum- 
stance connected  with  the  previous  returns  of  the 
comet,  which  seemed  to  Lalande  to  indicate  the 
possibility  of  the  return  of  the  comet  in  1759,  with- 
out being  seen.  In  1456  it  occupied  a  space  nearly 
seventy  degrees  in  length,  and  spread  terror  through- 
out Europe.  In  1607,  its  appearance,  as  described 
by  Kepler,  was  that  of  a  star  of  the  first  magnitude, 
and  so  trifling  was  its  tail  that  it  was  at  first  doubted 
whether  it  had  any.  In  1682  it  excited  very  little 
attention,  except  among  astronomers;  and  supposing 
this  decrease  of  magnitude  and  brilliancy  to  be  pro- 
gressive, Lalande  entertained  serious  apprehensions 
that  on  its  expected  return  it  might  escape  the  ob- 
servation even  of  astronomers ;  and  that  thus,  this 
splendid  example  of  the  power  of  science  and  un- 
answerable proof  of  the  principle  of  universal  gravi- 
tation, would  be  lost  to  the  world.  It  is  indeed  true, 
that,  in  1607  and  in  1682,  the  comet  was  not  favor- 
ably situated  in  respect  to  the  earth  and  sun  to  ap- 
pear with  great  brilliancy,  while  in  1456  its  position 
was  exactly  the  reverse  of  this.  But  still  it  did  not 
seem  hardly  possible  that  so  great  a  change  in  phy- 
sical appearance  could  be  caused  by  simply  the 
difference  of  position  with  respect  to  the  earth  and 


HALLE  Y  '  s    COMET.  197 

sun  at  the  time  of  its  visibility ;  and  it  may,  there- 
fore, be  said,  that  it  affords  the  very  greatest  interest 
to  observe  the  misgivings  of  this  distinguished  as- 
tronomer, with  respect  to  the  appearance  of  the  body 
according  to  the  prediction,  in  connection  with  his 
unshaken  faith  in  the  results  obtained.  He  asserted 
that  it  would,  without  doubt,  return  ;  and  that,  even 
if  astronomers  failed  to  see  it,  they  would  not,  on 
that  account,  be  less  convinced  of  its  presence. 
They  would  know  that  the  faintness  of  its  light,  its 
great  distance,  and,  perhaps,  even  an  unfavorable 
state  of  the  weather,  might  prevent  its  being  seen. 
The  world,  however,  would  find  it  difficult  to  be- 
lieve in  his  results,  and  would  place  this  discovery, 
which  had  done  so  much  honor  to  modern  philoso- 
phy, among  the  number  of  chance  predictions. 

Under  such  circumstances,  the  return  of  the  comet 
was  awaited  by  astronomers  with  the  greatest  im- 
patience and  anxiety.  They  were  anticipated,  how- 
ever, in  the  discovery,  by  George  Palitzch,  a  peasant, 
residing  in  the  neighborhood  of  Dresden.  He  first 
saw  the  comet  on  the  evening  of  the  25th  of  De- 
cember, 1758,  through  a  small  telescope,  and  on  the 
next  day  communicated  the  discovery  to  Hoffman, 
who  observed  it  on  the  27th  and  28th  of  December. 
A  few  days  later  it  was  independently  discovered  by 
an  astronomer  at  Leipsic,  who,  jealous  of  his  dis- 
covery, did  not  announce  it,  but  gave  himself  up  to 
17* 


198  TREATISE   ON    COMETS. 

the  solitary  pleasure  of  following  the  body  in  its 
course  from  day  to  day,  while  his  cotemporaries 
throughout  Europe  were  vainly  directing  their 
anxious  search  for  it  to  other  quarters  of  the  heavens. 
Delisle,  a  French  astronomer,  and  Messier,  his  as- 
sistant, had  been  constantly  engaged  in  searching 
for  it,  but  an  error  in  the  computation  of  the 
ephemeris,  by  the  former,  had  diverted  their  atten- 
tion to  a  different  part  of  the  heavens  from  that  in 
which  the  comet  was  to  be  seen ;  and  the  consequence 
was,  that  it  was  not  seen  by  Messier  until  the  21st 
of  January,  1759,  nearly  a  month  after  its  discovery 
by  Palitzch,  but  without  knowing  that  it  had  been 
already  observed. 

The  news  of  the  discovery  now  became  generally 
known,  and  the  comet  was  observed  at  various 
places  in  Europe.  On  its  first  appearance  the  nu- 
cleus wras  round  and  brilliant,  surrounded  by  a  vapor 
or  nebulous  atmosphere,  but  did  not  exhibit  any 
indications  of  a  tail.  It  was  rapidly  approaching 
the  sun,  and  about  the  middle  of  February  was  lost 
in  the  approaching  twilight.  It  passed  its  perihelion 
soon  after  midnight  at  Paris,  on  the  12th  of  March, 
1759,  only  twenty-three  days  before  the  time  pre- 
dicted by  Clairaut  and  Lalande,  or  within  the  limits 
of  probable  error  which  they  had  assigned  to  their 
results.  Laplace  has  since  shown,  that  if  the  mass 
of  Saturn  had  been  accurately  known  at  the  time 


H ALLEY'S   COMET.  199 

when  the  computations  were  made,  the  error  of  the 
final  result  would  not  have  exceeded  nine  days. 
This,  together  with  the  fact  that  the  planets  Uranus 
and  Neptune  have  since  been  discovered,  and  that 
the  existence  of  an  ethereal  fluid  pervading  the 
regions  of  space  has  been  established  by  its  action 
on  the  motions  of  the  comets,  enables  us  to  form  a 
true  conception  of  the  importance  of  Clairaut's 
labors,  which  had  revealed  the  place  of  the  long- 
expected  wanderer,  while  yet  invisible  to  the  naked 
eye,  on  its  return  after  an  absence  of  three-quarters 
of  a  century,  to  crown  with  triumph  the  illustrious 
astronomer  who  had  first  foretold  its  period.  What 
greater  exemplification  of  the  transcendant  powers 
of  the  human  understanding  could  be  desired,  or 
what  greater  exhibition  of  the  achievement  of 
genius?  An  object  which  had  once  caused  the 
greatest  alarm  throughout  all  Europe,  and  which 
had  been  supposed,  in  former  times,  to  have  caused 
some  of  the  greatest  disasters  which  have  ever  be- 
fallen the  human  race,  was  now  divested  of  its 
terrific  aspect  and  nature ;  and  hereafter,  as  it  re- 
turned successively  in  its  long  journey  through 
space,  to  be  hailed  with  increasing  delight,  as 
affording  new  and  conclusive  proofs  of  the  harmony 
of  those  beautiful  laws  which  govern  the  celestial 
motions. 

The  comet  emerged  from  the  sun's  rays  toward 


200  TREATISE   ON   COMETS. 

the  end  of  March,  1759,  and  was  visible  in  the 
morning  just  before  sunrise;  and,  had  it  not  been 
iii  the  bright  twilight,  was  in  the  most  favorable 
position,  with  reference  to  the  earth  and  sun,  for 
being  seen  in  all  its  splendor.  On  the  1st  of  April, 
Messier  was  able  to  distinguish  a  tail  with  the  aid 
of  a  telescope ;  but  in  no  other  instance  was  it  cer- 
tainly observed  in  Europe.  On  the  17th  of  April  it 
ceased  to  be  visible  in  the  morning ;  on  the  evening 
of  the  29th  of  the  same  month  it  appeared  about 
the  size  of  one  of  the  largest  stars,  and  from  the  3d 
of  June  ceased  to  be  visible.  The  apparent  dimi- 
nution of  the  comet  in  magnitude  and  brilliancy, 
already  noticed  by  Lalande,  may  here  seem  to  be 
positively  confirmed  as  a  .physical  fact,  since  its 
splendor  at  this  return  was  even  less  than  in  1682. 
But  it  should  be  remembered  that  it  was  seen  in 
1759  under  the  most  disadvantageous  circumstances. 
In  April,  when  the  tail  ought  to  have  been  longest, 
the  comet  was  far  from  the  earth,  and  rapidly  re- 
ceding from  the  sun ;  and,  moreover,  was  almost 
always  obscured  by  the  effect  of  the  twilight. 
Again,  it  may  be  readily  understood  why  a  comet 
may,  at  successive  returns  to  our  system,  sometimes 
appear  to  have  a  tail,  and  sometimes  to  be  without 
one,  according  to  its  position  with  regard  to  the 
earth  and  sun.  It  may  happen  that,  at  one  return, 
the  earth  may  be  near  the  comet  at  or  near  the  time 


H  A  L  L  E  Y  '  S    XJ  0  M  E  T.  201 

of  its  perihelion  passage,  when  its  light  is  neces- 
sarily greatest  and  its  train  most  extended,  and  thus 
a  most  favorable  opportunity  will  be  afforded  for 
witnessing  its  physical  appearance ;  while,  at  its 
next  return,  the  earth  being  at  a  remote  part  of  its 
orbit  while  the  comet  is  passing  the  sun,  it  may  be 
seen  only  with  great  difficulty,  or  even  become 
quite  invisible.  It  is  evident,  therefore,  that  no 
expectation  of  a  uniform  physical  appearance  of  a 
comet  at  successive  returns,  can  be  indulged ;  and 
that  there  is  scarcely  the  slightest  chance  of  ever 
being  able  to  recognize  even  a  single  one  among 
the  many  thousands  which  are  sweeping  through 
the  regions  of  space,  by  means  of  any  apparent 
similarity  of  form  and  brilliancy. 

In  the  southern  hemisphere,  Halley's  comet  was 
much  more  favorably  situated  for  observation,  in 
1759,  than  in  the  higher  latitudes  of  the  northern 
hemisphere.  It  was  observed  at  the  Isle  of  Bourbon 
and  at  Pondicherry,  and  the  tail  was  distinctly  visi- 
ble to  the  naked  eye  at  both  places,  its  length 
varying  from  ten  to  forty-seven  degrees,  which 
seems  to  accord  fully  with  the  former  appearances 
of  the  same  body. 

Such  were  the  circumstances  connected  with  the 
first  predicted  return  of  Halley's  comet;  and  its 
appearance,  in  accordance  with  the  prediction,  ex- 
cited the  curiosity  and  Avonder  of  both  the  learned 


202  TREATISE   ON    COMETS. 

and  unlearned  throughout  all  Europe.  Another 
interval  of  seventy-six  years  has  elapsed  since  its 
appearance  in  1759,  the  same  computations  have 
been  performed,  only  that  this  time  the  planet  Ura- 
nus was  known,  and  the  mass  of  Saturn  had  been  ac- 
curately determined;  and  on  the  16th  of  November, 
1835,  the  comet,  true  to  its  predicted  course,  passed 
the  perihelion  of  its  orbit.  This  revolution,  how- 
ever, was  performed  under  circumstances  far  more 
favorable  than  had  ever  before  occurred.  During 
the  interval  between  1759  and  1835,  the  science  of 
analysis,  as  applied  to  physical  astronomy,  had 
made  rapid  advances.  The  methods  of  investigation 
had  acquired  greater  simplicity,  and  had  also  been 
made  more  general  and  comprehensive.  Learned 
societies  had  likewise  been  established  in  several 
cities  in  Europe  and  elsewhere ;  and  had  stimulated 
the  spirit  of  inquiry  by  a  succession  of  prizes 
offered  for  the  solution  of  problems  arising  out  of 
the  difficulties  which  were  progressively  developed 
by  the  advancement  of  astronomical  science.  Among 
these  questions,  the  determination  of  the  elliptic 
orbits  of  comets,  and  the  perturbations  which  they 
experience  in  their  course,  by  the  action  of  the 
planets  near  which  they  happen  to  pass,  were  con- 
sidered of  primary  importance.  The  French  Aca- 
demy, accordingly,  offered,  in  the  year  1778,  a  high 
mathematical  prize  for  an  essay  on  this  subject. 


H ALLEY'S    COMET.  203 

which  called  forth  a  Memoir  from  Lagrange,  in 
which  he  formed  at  once  a  complete  solution  and  a 
model  for  all  future  investigations  of  the  same  kind. 
This  investigation  by  Lagrange  was,  however,  of  a 
general  character;  and  it  remained  to  apply  it  to  the 
particular  case  of  Halley's  comet,  the  only  one  then 
known  whose  periodicity  had  been  definitely  deter- 
mined. In  1820,  the  Academy  of  Sciences  at  Turin 
offered  a  prize  for  this  application  of  Lagrange's 
theory,  which  was  awarded  to  Damoiseau,  a  French 
astronomer ;  and,  in  1826,  the  French  Institute  pro- 
posed a  similar  prize,  having  offered  it  twice  before 
Avithout  calling  forth  any  claimant.  This  prize  was 
awarded  to  Pontecoulant,  who  determined  the  per- 
turbations of  Halley's  comet  by  taking  into  account 
the  simultaneous  action  of  Jupiter,  Saturn,  Uranus, 
and  the  earth  —  the  comet  having,  in  1759,  passed 
sufficiently  near  the  latter  to  experience  from  it  sen- 
sible disturbances.  He  found,  finally,  that  it  would 
pass  its  perihelion  on  the  7th  of  November,  1835 ; 
and  afterwards,  by  a  revision  of  the  calculations,  he 
predicted  that  its  arrival  at  that  point  of  its  orbit 
nearest  the  sun,  would  take  place  on  the  morning 
of  the  14th  of  the  same  month. 

The  same  problem  was  undertaken  by  Lubbock, 
Rosenberger,  and  Lehmann.  Damoiseau  and  Ponte- 
coulant had  assumed  the  orbit  in  which  the  comet 
was  moving  in  1759  as  the  basis  of  their  investiga- 


204  TREATISE   ON    COMETS. 

tions,  and  had  calculated  simply  the  alterations 
which  would  be  produced  upon  it  by  the  action  of 
the  planets  from  1759  to  1835.  In  his  second  solu- 
tion, Pontecoulant  corrected  the  elements  by  means 
of  the  observations  in  1759,  and  consequently  ob- 
tained more  satisfactory  results.  Lubbock,  Rosen- 
berger,  and  Lehmann,  undertook  in  the  first  place 
to  ascertain  the  orbit  which  it  followed  in  1759,  by 
means  of  such  observations  as  had  been  recorded  at 
that  time ;  and  then,  by  making  due  allowance  for 
the  planetary  perturbations,  to  predict  the  exact 
date  of  its  return  to  its  perihelion.  It  results,  there- 
fore, as  might  be  expected,  that  the  time  of  passing 
the  perihelion  in  1835,  as  found  by  these  astrono- 
mers, exhibits  a  considerable  uncertainty  in  the 
period,  arising  from  their  having  adopted  different 
systems  of  elliptic  elements  of  the  orbit  of  the  comet 
at  its  former  appearance.  The  time  of  perihelion 
passage  obtained  by  the  astronomers  above  named, 
are  as  follows : 

Lubbock        ....     October      31,  1835. 
Damoiseau     ,  November    4,     " 

Rosenberger          .        ,       .Tv  11,     " 

Pont6coulant         .         .         .  "  14,     " 

Lehmann       .         .         .'        *  ;          "  26,     " 

The  computations  of  Rosenberger  and  Lehmann 
were  published  only  a  short  time  before  the  comet 
made  its  appearance,  —  those  of  Lehmann  having 
been  announced  on  the  25th  of  July,  1835.  They 


HALLE Y'S   COMET.  205 

published  also  an  ephemeris  of  the  comet  for  the 
period  of  its  visibility,  in  which  its  exact  route  in 
the  heavens  was  accurately  designated,  and  the  date 
at  which  it  would  probably  be  visible  with  the  aid 
of  a  telescope.  On  the  5th  of  August,  1835,  Du- 
mouchel,  Director  of  the  Observatory  of  the  Roman 
College,  at  Rome,  directed  his  telescope  to  the  point 
indicated  by  Rosenberger's  ephemeris ;  and  saw  the 
comet,  as  a  faint  and  almost  invisible  stain  of  light 
on  the  deep  blue  of  the  heavens,  within  a  degree  of 
its  predicted  place.  On  the  20th  of  August  it  was 
generally  observed  in  Europe,  and  was  subsequently 
followed  until  about  the  time  of  its  perihelion  pass- 
age, on  the  16th  of  November,  when  it  ceased  to  be 
visible,  on  account  of  its  having  moved  toward  the 
south.  It  was  observed  in  the  southern  hemisphere 
throughout  February,  March,  and  April,  1836;  and 
disappeared  finally  on  the  17th  of  May  of  that  year. 
When  the  comet  first  became  visible,  it  presented 
the  appearance  of  a  small,  round,  or  somewhat  ellip- 
tic nebula,  without  any  indications  of  a  tail,  and 
having  a  point  in  which  the  light  seemed  to  be 
strongly  condensed,  situated  eccentrically  within  it. 
On  the  2d  of  October  the  first  indications  of  a  tail 
were  exhibited,  which  increased  rapidly,  and  on  the 
5th  had  attained  a  length  of  five  degrees.  On  the 
15th  of  October,  nearly  a  month  before  its  perihelion 
passage,  the  tail  had  attained  its  maximum  length, 
18 


206  TREATISE   ON    COMETS. 

which  was  upwards  of  twenty  degrees.  From  that 
date  it  decreased  rapidly,  and  on  the  29th  of  Octo- 
ber it  was  only  three  degrees  in  length.  It  con- 
tinued to  decrease ;  and  on  the  16th  of  November, 
the  date  of  its  perihelion  passage,  the  comet  was 
observed  at  Pulkova,  in  Russia,  where  no  tail  was 
visible. 

Previous  to  the  return  of  the  comet,  although  its 
physical  appearance  was  not  expected  to  be  such  as 
to  create  sensations  of  terror,  even  among  those 
inclined  to  be  superstitious,  yet  the  expectation  was 
very  general  among  astronomers  that,  at  its  return 
to  its  perihelion  in  1835,  it  would  afford  an  oppor- 
tunity for  obtaining  new  data,  from  which  to  derive 
some  satisfactory  theory  respecting  the  physical  con- 
stitution of  the  class  of  bodies  of  which  it  is  so 
striking  an  example.  It  is  perhaps  unnecessary  to 
state,  that  it  no  sooner  became  visible  than  pheno- 
mena began  to  be  exhibited,  preceding  and  accom- 
panying the  gradual  formation  of  the  tail,  the  obser- 
vation of  which  has  been  most  justly  regarded  as 
forming  a  memorable  era  in  the  history  of  astrono- 
my,—  or  more  especially  in  that  relating  to  the 
cometary  bodies.  All  those  strange  and  important 
appearances  which  the  comet  presented  were  ob- 
served with  the  greatest  zeal,  and  delineated  with 
the  utmost  fidelity,  by  several  different  astronomers, 
among  whom  were  Bessel,  Schwabe,  and  Struve,  in 


H ALLEY'S   COMET.  207 

the  northern  hemisphere,  mid  Maclear  and  Herschel 
at  the  Cape  of  Good  Hope ;  each  of  whom  have 
recorded  the  successive  transformations  presented 
hy  the  comet,  under  the  physical  influence  of  a  con- 
stantly varying  temperature,  in  its  approach  to  and 
departure  from  the  sun. 

The  most  striking  phenomena  which  the  comet 
presented  early  in  October,  were  those  which,  com- 
mencing simultaneously  with  the  growth  of  the  tail, 
evidently  connected  themselves  with  the  production 
of  that  appendage,  and  its  projection  from  the  head 
of  the  comet.  On  the  2d  of  October,  the  nucleus, 
which  had  hitherto  been  faint  and  small,  was  ob- 
served to  have  become  suddenly  brighter,  and  clearly 
indicated  the  commencement  of  the  formation  of 
the  tail,  by  the  appearance  of  being  in  the  act  of 
throwing  out  a  jet  or  stream  of  light  from  that  part 
presented  toward  the  sun.  This  ejection  was  by 
no  means  either  uniform  or  continuous,  but  appeared 
to  take  place  at  intervals,  like  the  issuing  of  the 
fiery  matter  from  the  crater  of  a  volcano.  On  the 
4th  of  October,  as  seen  through  a  powerful  telescope, 
the  central  part  of  the  comet  presented  a  very  curi- 
ous appearance.  The  nucleus  was  elliptical,  at  least 
five  or  six  times  longer  in  one  direction  than  in  the 
other;  and  is  said  to  have  resembled  a  burning  coal, 
from  which  there  issued,  in  a  direction  nearly  oppo- 
site to  that  of  the  tail,  a  divergent  flame,  varying  in 


208  TREATISE   ON    COMETS. 

intensity,  in  form,  and  in  direction.  Occasionally  it 
appeared  doubled,  and  seemed  to  indicate  that  a 
luminous  gas  was  issuing  from  the  nucleus.  This 
ejection,  after  ceasing  for  a  brief  period,  was  re- 
sumed on  the  8th  of  October,  and  continued,  much 
more  violently  than  before,  with  occasional  inter- 
ruptions, until  the  tail  ceased  to  be  visible.  At  this 
time,  Schwabe  noticed  what  he  considered  to  be  a 
secondary  train,  extending  for  a  short  distance  in  a 
direction  opposite  to  that  of  the  original  tail,  and, 
therefore,  toward  the  sun.  Bessel,  however,  re- 
garded this  appearance  merely  as  the  renewed  ejec- 
tion of  nebulous  matter  which  was  afterwards  turned 
back  from  the  sun,  as  smoke  would  be  by  a  current 
of  air  blowing  from  the  sun  in  the  direction  of  the 
real  tail.  Both  the  form  of  this  luminous  ejection, 
and  the  direction  in  which  it  issued  from  the  nu- 
cleus, were  observed  to  undergo,  from  time  to  time, 
singular  and  capricious  alterations  —  the  different 
phases  so  rapidly  succeeding  each  other,  that  on  no 
two  successive  evenings  were  the  appearances  simi- 
lar. Sometimes  there  were  two,  and  often  three, 
nebulous  emanations  observed  to  issue  from  the 
nucleus  in  divergent  directions.  The  directions  of 
tjiese  emanations  of  nebulous  matter,  and  also  their 
comparative  brightness,  appeared  to  be  subject  to 
continual  variations.  At  one  time  the  emitted  jet 
was  observed  to  be  single,  as  above  described,  and 


II  A  L  L  E  Y  '  s   COMET.  209 

confined  within  narrow  limits  of  divergence  from 
the  nucleus.  At  other  times,  it  presented  a  fan- 
shaped  or  swallow-tailed  form,  not  unlike  that  of 
the  flame  issuing  from  a  gas-burner  with  a  flattened 
orifice. 

Appearances  similar  to  these  were  noticed  by 
Arago,  at  Paris,  and  also  by  other  astronomers  at 
several  different  places.  On  the  15th  of  October, 
Arago  saw  a  luminous  sector,  or  diverging  light, 
issuing  from  the  head  of  the  comet,  a  little  to  the 
south  of  the  point  immediately  opposite  the  tail, 
which  was  much  more  brilliant  than  the  rest  of  the 
nebulosity,  and  was  bounded  by  two  lines  of  fainter 
light  directed  toward  the  centre  of  the  head.  On 
the  following  evening  no  trace  of  this  sector  ex- 
isted ;  but  at  a  point  diametrically  opposite  the  axis 
.of  the  tail,  a  new  sector  was  formed,  of  more  than 
a  quarter  of  a  circumference,  in  angular  extent, 
which  was  much  more  elongated  and  brilliant  than 
the  one  first  seen,  and  which  was  bounded  by  two 
very  bright  lines  tending  to  the  centre  of  the  head. 
On  the  17th  this  appearance  had  diminished  in 
splendor,  and  on  the  evening  of  the  18th  —  the 
atmosphere  being  extremely  clear  —  the  entire 
comet,  including  the  tail,  had  very  sensibly  de- 
creased in  brilliancy.  On  the  21st,  there  were  three 
luminous  sectors  or  brushes  of  light  seen  in  the 
nebulosity  of  the  head,  the  most  feeble  and  dilated 
18*  o 


210  TREATISE   ox    COMETS. 

of  which  was  exactly  in  the  prolongation  of  the  tail. 
Two  days  later,  these  had  totally  vanished,  but  the 
whole  aspect  of  the  comet  was  completely  and 
suddenly  changed.  The  nucleus,  hitherto  so  bril- 
liant and  well-defined,  had  become  large  and  dif- 
fused ;  and  although  it  still  occupied  the  centre  of 
the  head,  yet  the  brilliancy  of  the  nebulosity  on  the 
eastern  side  far  surpassed  that  on  the  western. 

Another  remarkable  circumstance  connected  with 
the  emission  of  the  luminous  jets  already  described, 
was  that  the  direction  of  the  principal  jet  was  ob- 
served to  oscillate  to  and  fro,  on  either  side  of  a  line 
drawn  from  the  sun  through  the  centre  of  the  head 
of  the  comet,  just  as  the  needle  of  a  compass  oscil- 
lates on  either  side  of  the  magnetic  meridian.  This 
vibratory  motion  was  so  rapid  that  the  direction  of 
the  jets  was  observed  to  be  visibly  changed  from 
hour  to  hour,  under  the  eye  of  the  observer.  These 
jets,  though  very  bright  at  their  point  of  emanation 
from  the  nucleus,  are  described  as  fading  rapidly 
away,  and  becoming  diffused  as  they  expanded  into 
the  coma,  at  the  same  time  curving  backwards  like 
streams  of  steam  or  smoke,  thrown  out  more  or  less 
obliquely  from  narrow  orifices,  in  opposition  to  a 
powerful  wind,  against  which  they  are  unable  to 
make  way,  and  ultimately  yielding  to  its  force,  are 
drifted  back  and  confounded  in  a  vaporous  train, 
following  the  general  direction  of  the  current.  At 


II  ALLEY'S   COMET.  211 

one  time,  Struve  saw  the  comet  thus  attended  by 
two  delicately-shaped  appendages  of  light,  of  a  most 
graceful  form,  one  of  which  preceded,  and  the  other 
followed  the  nucleus.  Subsequently  it  appeared  to 
be  surrounded  by  a  sort  of  semicircular  veil,  which, 
extending  backward,  was  lost  in  a  double  train  of 
light,  stretching  out  to  a  vast  distance  from  the 
body  of  the  comet.  On  the  5th  of  November,  1835, 
when  seen  through  the  powerful  telescope  at  Pul- 
kova,  two  flames  were  seen  issuing  from  the  nucleus 
in  nearly  opposite  directions,  both  of  which  were 
curved  toward  the  same  side.  The  brighter  flame, 
directed  toward  the  north,  was  marked  by  strongly 
defined  edges;  while  the  other,  directed  toward 
the  south,  was  more  feeble  and  ill-defined.  The 
opposite  diagram  represents  the  comet  as  seen  at 
this  time. 

The  comet  was  not  observed  in  Europe,  with  but 
few  exceptions,  after  the  perihelion  passage.  For 
upwards  of  two  months  it  was  lost  in  the  sun's 
rays,  but  again  reappeared  on  or  about  the  24th  of 
January,  1836,  when  it  was  observed  in  the  southern 
hemisphere.  Its  aspect,  however,  was  altogether 
different  from  that  under  which  it  had  been  seen 
previous  to  its  perihelion  passage  —  having,  during 
the  interval  in  which  it  was  invisible,  undergone 
some  great  physical  change,  .which  had  operated  to 
produce  an  entire  transformation  of  its  physical 


212  TREATISE   ox    COMETS. 

appearance.  It  is  described  as  no  longer  presenting 
any  vestige  of  a  tail,  but  as  appearing  to  the  naked 
eye  about  as  bright  as  a  star  of  the  fifth  magnitude, 
shining  through  a  haze ;  while,  with  the  aid  of  a 
powerful  telescope,  it  was  seen  as  a  planetary 
nebula,  with  a  small,  round,  and  well-defined  disc, 
surrounded  by  a  nebulous  envelope  or  coma  of  con- 
siderable extent.  Within  the  well-defined  head  or 
disc,  and  somewhat  eccentrically  placed,  was  a  vivid 
nucleus,  resembling  a  miniature  comet,  with  a  head 
and  tail  of  its  own,  perfectly  distinct  from  and 
greatly  exceeding,  in  the  intensity  of  its  light,  the 
other  portions  of  the  comet.  A  minute  bright 
point,  resembling  a  small  star,  was  distinctly  per- 
ceived within  it,  but  which  was  never  sufficiently 
well  defined  to  give  any  positive  assurance  of  the 
existence  of  a  solid  sphere,  nor  could  any  indication 
of  a  gibbous  form  be  detected. 

The  phenomena  and  changes  which  the  comet 
presented  subsequent  to  its  reappearance  in  the 
southern  hemisphere,  and  until  its  final  disappear- 
ance, were  carefully  observed  at  the  Cape  of  Good 
Hope.  As  it  receded  from  the  sun,  the  coma  or 
envelope  speedily  disappeared,  as  if  absorbed  into 
the  disc,  which,  on  the  contrary,  is  said  to  have  in- 
creased continually  in  dimensions,  and  with  such 
rapidity,  that  in  the  week  ending  February  1st, 
1836,  the  actual  volume  or  real  solid  contents  of  the 


II ALLEY'S   Co M  E T.  213 

illuminated  space  had  expanded  at  least  forty  times. 
This  increase  of  the  size  of  the  nucleus,  or  rather 
the  head,  continued  with  undiminished  rapidity 
until  the  final  disappearance  of  the  comet.  The 
brilliancy  of  the  comet  was  found  to  decrease  in 
proportion  as  the  magnitude  of  the  head  increased. 
The  nucleus  of  the  comet,  properly  so  called,  was 
not,  however,  observed  to  undergo  any  very  con- 
siderable change,  while  the  envelope  was  continually 
dilating,  and  becoming  fainter  and  fainter.  More- 
over, the  ray  proceeding  from  it  is  said  to  have  in- 
creased in  length  and  comparative  brightness,  pre- 
serving its  direction  along  the  greater  diameter  of 
the  head,  —  which  had  acquired  the  form  of  a 
parabola,  —  and  exhibiting  none  of  those  irregular 
phenomena  which  characterized  the  jets  emitted 
previous  to  its  perihelion  passage.  This  ray  of 
nebulous  light,  however,  faded  away  by  degrees; 
and  on  the  5th  of  May  the  comet  appeared  precisely 
as  at  the  time  of  its  first  appearance  in  August, 
1835,  or  as  a  small  round  nebula,  with  a  condensa- 
tion of  light  near  its  centre.  This  was  the  last 
observation  of  the  comet  in  the  southern  hemi- 
sphere ;  but  it  was  seen  twelve  days  later  by  Lamont, 
at  Munich,  in  Bavaria,  when  it  disappeared  not  to 
return  again  until  about  the  end  of  February,  1911. 
The  position  of  the  comet  in  the  heavens  from 
day  to  day  was  also  observed  with  great  precision ;  and 


214  TEE-ATISE   ON   COMETS. 

from  these  observations,  combined  with  those  made 
in  1759,  and  at  previous  returns,  it  will  now  be  pos- 
sible to  investigate  all  the  circumstances  of  its  mo- 
tion, and  to  predict  the  date  of  its  next  perihelion 
passage  within  a  few  hours,  or,  perhaps,  within  less 
than  an  hour.  To  accomplish  this  will  require  im- 
mense labor,  being  sufficient,  when  carefully  per- 
formed, to  occupy  the  entire  time  of  one  computer 
for  several  years.  But  great  as  this  task  may  seem 
to  be,  it  is  nevertheless  certain  that  some  one  will 
undertake  it — if  it  has  not  been  commenced  already 
— and  that  the  time  of  the  next  return  of  the  comet 
will  be  so  accurately  predicted,  that  the  exact  place 
in  the  heavens  to  which  the  telescope  must  be 
directed  will  be  announced,  and  the  comet  will  be 
seen  in  strict  accordance  with  the  announcement. 
It  is  indeed  true,  that  there  may  be  bodies  of  a 
planetary  or  cometary  nature  in  the  far  distant 
regions  of  the  heavens  in  which  the  comet  wanders, 
which  may  disturb  its  motions  and  change  the  period 
of  its  revolution ;  but  so  accurately  can  the  disturb- 
ing influence  of  all  the  known  bodies  of  our  system 
be  calculated,  that  the  existence  of  these  unknown 
sources  of  perturbation  will  be  made  evident  to  future 
astronomers,  should  any  difference  be  hereafter  ex- 
hibited between  the  computed  and  actual  orbit  of 
the  comet. 

The  accuracy  of  Pontecoulant's  prediction  of  the 


II  A  L  L  E  Y  '  s   COMET.  215 

perihelion  passage  of  the  comet  in  1835,  affords  a 
striking  instance  of  the  precision  of  the  methods  of 
calculation  employed  by  him,  when  we  consider  the 
long  period  of  seventy-five  years  during  which  the 
comet  had  been  invisible.  Moreover,  the  small  dis- 
crepancy of  only  two  days  can  be  readily  accounted 
for  by  the  fact  that  the  existence  of  the  planet  Nep- 
tune was  then  hardly  suspected,  and  the  masses  of 
Jupiter  and  Uranus  were  not  then  as  well  deter- 
mined as  at  the  present  time.  And,  further,  the  ex- 
istence of  a  medium  of  resistance  has  since,  as  we 
shall  see,  been  conclusively  established.  The  influ- 
ence of  this  ethereal  medium  on  the  motion  of  this 
comet  cannot  be  determined,  with  great  certainty, 
until  it  has  performed  another  revolution.  Its  effect 
on  the  duration  of  the  last  period  of  the  comet  has 
indeed  been  computed  ;  but  this  is  a  problem  which, 
in  the  present  state  of  our  knowledge,  can  be  solved 
only  by  experience,  since  it  is  evident  that  the  effect 
of  such  a  medium  of  resistance  as  is  here  supposed 
to  exist,  must  depend  upon  the  magnitude  and 
density  of  the  comet,  and  upon  the  law  by  which 
the  density  of  the  ethereal  fluid  varies  with  the  dis- 
tance from  the  sun.  It  is  supposed  that  this  medium 
has  acquired  a  rotary  motion  from  east  to  west  — 
especially  within  the  limits  of  the  solar  system — and 
it  will  therefore  be  necessary,  in  order  to  arrive  at 
results  of  the  very  greatest  accuracy,  to  decide  what 


216  TREATISE   ON    COMETS. 

part  of  the  variations  in  the  period  of  its  revolution 
may  be  due  to  that  cause.  These  and  similar  ques- 
tions are  to  be  carefully  discussed  before  a  precise 
knowledge  of  the  motions  of  the  comet  can  be  ob- 
tained; but  that  all  this,  and  even  more,  will  be 
accomplished,  the  history  of  astronomy  during  the 
last  fifty  years  furnishes  ample  reason  to  expect. 

The  influence  of  the  attraction  of  the  planets  on 
the  motions  of  Halley's  comet,  can  be  computed  with 
extreme  precision ;  and  were  there  no  other  disturb- 
ances to  be  experienced  during  the  entire  course  of 
its  revolution  around  the  sun,  it  would  be  possible 
to  predict  its  future  movements  with  the  same  pre- 
cision in  which  the  motions  of  the  planets  are  de- 
termined. It  is  possible  that  the  orbit  of  Neptune 
does  not  mark  the  confines  of  the  planetary  system, 
but  that  there  may  be  other  planets  still  more 
remote,  which,  in  due  course  of  time,  may  change 
the  motions  of  the  comet,  and  which,  in  case  they 
are  not  sooner  revealed,  may  finally  cause  it  to  fail 
to  appear  in  accordance  with  prediction.  The  time 
will  indeed  come,  when  the  action  of  Neptune  will 
greatly  affect  the  orbit  of  the  comet ;  and  had  this 
planet  not  been  discovered,  it  would  at  some  future 
day  have  falsified  the  predictions  of  astronomers, 
and  would,  perhaps,  have  again  involved  the  entire 
theory  of  cometary  motions  in  impenetrable  mystery. 
It  is  with  no  small  degree  of  pleasure,  therefore, 


HALLEY'S   COMET.  217 

that  the  corresponding  advancement  of  every  de- 
partment of  physical  science  can  be  contemplated. 
The  discovery  of  Neptune  was  itself  one  of  the  most 
brilliant  achievements  of  the  human  intellect  which 
has  been  recorded ;  but  it  is  not  until  its  entire  influ- 
ence in  the  system  is  considered,  that  we  can  form 
a  true  conception  of  its  ultimate  importance. 

Some  idea  may  be  formed  of  the  vast  size  of  the 
orbit  of  Halley's  comet,  by  comparing  it  with  the 
dimensions  of  that  of  our  earth.  The  mean  distance 
of  the  earth  from  the  sun  is  about  95,000,000  miles, 
while  the  length  of  the  orbit  of  the  comet  is  about 
thirty-six  times  this  distance,  and  its  breadth  about 
one-fourth  of  its  length.  It  should  be  remembered, 
however,  that  the  eccentricity  of  the  orbit  of  the 
earth  is  very  slight,  while  that  of  the  comet  is  very 
great ;  and,  consequently,  the  distance  of  the  comet 
from  the  sun  is  subject  to  great  variations.  In 
approaching  the  sun,  its  velocity  continually  in- 
creases, until  it  darts  around  him  with  astonishing 
rapidity,  at  a  distance,  when  in  the  perihelion,  of 
only  47,000,000  miles  from  its  centre.  Its  velocity 
then  gradually  diminishes,  after  leaving  the  sun,  till 
it  reaches  the  most  remote  point,  or  aphelion  of 
its  orbit,  where  its  distance  from  him  is  about 
3,373,000,000  miles  — a  distance  which  exceeds  that 
of  the  remotest  planet  of  our  system,  but  at  which 
the  attraction  of  the  sun  is  still  sufficient  to  recall 
19 


218  TREATISE   ON    COMETS. 

the  comet  toward  him.  When,  therefore,  the  comet 
is  in  its  perihelion,  the  amount  of  heat  and  light 
which  it  will  receive  from  the  sun,  will  be  four  times 
greater  than  what  is  received  at  the  earth.  When 
the  comet  is  in  the  aphelion  of  its  orbit,  the  light 
and  heat  will  be  nearly  6000  times  less  than  at  the 
perihelion ;  and  could  the  sun  be  viewed  from  the 
comet  at  this  enormous  distance,  it  would  not  appear 
larger  than  a  star.  It  is  evident,  therefore,  that  the 
vicissitudes  of  temperature  experienced  by  the  comet 
must  be  almost  beyond  our  conception.  If  the  earth 
could  be  transported  to  the  aphelion  of  the  comet's 
orbit,  every  liquid  substance  would  become  solid  by 
congelation,  even  if  the  atmospheric  air  and  other 
permanent  gases  did  not  become  liquids  by  the  same 
process.  But  if,  on  the  other  hand,  the  earth  were 
placed  at  the  perihelion  of  the  orbit  of  the  comet, 
all  the  liquids  upon  it  would  be  converted  into 
vapor,  either  mixing  with  the  atmospheric  air,  or 
arranging  themselves  in  regular  strata,  one  above 
the  other,  according  to  their  relative  weights. 

We  have  thus  described,  somewhat  in  detail,  the 
most  important  events  connected  with  the  discovery 
of  the  periodic  character  of  Halley's  comet,  and  also 
the  most  interesting  phenomena  which  it  presented. 
We  have  seen  that  it  exhibited  to  a  high  degree 
those  physical  phenomena  which  seem  to  belong 
almost  exclusively  to  this  class  of  bodies,  and  that  it 


E  x  c  K  E  '  s   COMET.  219 

affords  an  unparalleled  example  of  the  result  of  those 
powers  of  calculation  by  which  we  are  enabled  to 
follow  it,  in  the  mind,  through  the  depths  of  space, 
far  beyond  the  extreme  limits  of  our  system,  and. 
notwithstanding  disturbances  which  render  each 
succeeding  period  of  its  return  different  from  the 
last,  to  foretell  that  return  with  the  utmost  precision. 
The  next  comet,  whose  period  of  revolution  has 
been  definitely  determined,  which  we  shall  consider, 
is  that  known  as  Encke's  comet,  and  which,  although 
one  of  the  most  interesting  in  its  phenomena,  is 
still,  in  almost  every  respect,  unlike  that  which  we 
have  just  described.  This  comet  was  discovered  by 
Pons,  at  Marseilles,  on  the  26th  of  November,  1818. 
It  was  not  visible  to  the  naked  eye  ;  and  when  seen 
through  a  telescope,  presented  the  appearance  of  an 
ill-defined  nebulosity,  without  a  tail,  but  evidently 
having  a  point  in  which  the  light  was  strongly  con- 
densed, situated  eccentrically  within  it.  As  soon  as 
it  had  been  observed  to  a  sufficient  extent,  the  ele- 
ments of  its  orbit  were  computed ;  and  on  comparison 
of  the  elements  of  those  comets  which  had  previously 
appeared,  it  was  immediately  noticed  that  this  comet 
was  identical  with  one  which  was  visible  in  1805. 
It  was  uncertain,  however,  whether  the  period  was 
thirteen  years,  or  whether  the  comet  had,  in  the 
mean  time,  returned  unperceived  to  its  perihelion. 
Subsequently,  Encke  undertook  to  remove  this 


220  TREATISE   ON   COMETS. 

doubt,  by  computing  an  elliptic  orbit  directly  from 
the  observations  made  in  1818  and  1819,  during  the 
entire  period  in  which  the  comet  was  visible  ;  and  he 
found  that  its  period  of  revolution  was  only  about 
1200  days,  and  that,  consequently,  the  comet  had 
returned  three  times  between  the  years  1805  and 
1819.  It  was  found,  also,  upon  further  examination, 
that  the  same  comet  had  been  observed  in  1786  and 
in  1795.  Having  thus  identified  the  comet  at  four 
different  returns  to  its  perihelion,  En  eke  was  enabled 
to  ascertain  the  period  of  its  revolution  with  great 
precision ;  the  result  being  1208  days.  This,  how- 
ever, occasioned  great  astonishment,  and  some  de- 
gree of  doubt,  since  it  was  generally  believed  that 
the  periods  of  comets  must  necessarily  be  very  long. 
The  identification  of  the  comet  with  those  of 
1786, 1795,  and  1805,  enabled  Encke  to  use  observa- 
tions including  an  interval  of  nearly  thirty-three 
years  in  making  his  computations ;  and  having  com- 
puted its  motion  from  the  year  1819,  he  predicted 
that,  as  its  period  would  be  reduced  to  1203  days  by 
the  disturbing  action  of  the  planets,  it  might  be 
expected  to  appear  about  the  beginning  of  June, 
1822,  but  that,  on  account  of  its  position  in  the 
heavens,  it  would  only  be  visible  in  the  southern 
hemisphere.  The  return  of  the  comet  was  therefore 
looked  for  by  the  astronomers  who  happened  to  be 
in  that  part  of  the  globe ;  and  on  the  2d  of  June, 


E  x  c  K  E  '  s   COMET.  221 

1822,  it  was  actually  discovered  by  Rumker,  at  Para- 
mata,  in  New  South  Wales,  and  observed  until  the 
end  of  the  same  month.  The  next  return  was  pre- 
dicted to  take  place  in  1825 ;  and  on  the  13th  of 
July — true  to  its  appointed  course — the  comet  was 
observed  by  Yalz,  at  Msmes.  The  next  return  took 
place  in  1828.  It  was  first  seen  by  Strove,  at  Dor- 
pat,  in  Russia,  on  the  13th  of  October  of  that  year, 
and  was  observed  in  the  European  observatories  till 
December  25th. 

In  1832,  the  comet  again  returned  to  its  perihelion ; 
but  being  unfavorably  situated  for  observation  in 
the  northern  hemisphere,  it  was  only  seen  by  Hard- 
ing, at  Gottingen,  on  the  21st  of  August.  It  was 
observed,  however,  by  Henderson,  at  the  Cape  of 
Good  Hope,  during  the  entire  month  of  June,  and 
was  also  seen  at  Buenos  Ayres.  In  1835  it  was  ob- 
served from  July  22d  till  August  6th,  and  in  1838  it 
was  seen  at  Breslau,  on  the  14th  of  August,  as  a 
very  faint,  ill-defined  object.  It  subsequently  in- 
creased in  brilliancy,  and  continued  visible  until  the 
middle  of  December.  In  combining  all  the  observa- 
tions which  have  been  made  from  1786  to  1838,  in- 
clusive, Encke  found  it  necessary  to  change  the 
value  which  he  had  hitherto  employed  for  the  mass 
of  the  planet  Mercury,  in  computing  the  perturba- 
tions of  the  comet,  and  also  to  make  allowance  for 
the  supposed  influence  of  a  resisting  ethereal  fluid, 
19* 


222  TREATISE   ON   Co M  E T s . 

pervading  the  regions  of  space,  as  already  alluded 
to  in  the  case  of  Halley's  comet.  It  should  be  re- 
marked, however,  that  even  before  this,  the  comet 
had  indicated  that  a  correction  to  the  adopted  value 
of  the  mass  of  Jupiter  was  necessary.  The  plane 
of  the  orbit  of  the  comet  being  inclined  to  that  of 
the  orbit  of  Jupiter  by  a  small  angle,  it  necessarily 
approaches  very  near  the  planet,  and,  consequently, 
is  subject  to  considerable  disturbance,  in  its  motions, 
from  this  source.  Under  these  circumstances  it  may 
be  perceived  that,  since  the  action  of  Jupiter  on  the 
comet  depends  on  their  distance  and  relative  quan- 
tities of  matter,  any  error  in  the  assumed  value  of 
the  mass  of  the  former  would  be  exhibited  by  the 
excess  or  deficiency  of  the  resulting  values  of  the 
perturbations.  But  Laplace  had  computed,  from 
the  theory  of  probabilities,  that  there  was  hardly 
any  chance  that  the  mass  of  the  planet,  which  was 
generally  used,  could  be  in  error  even  by  the  one- 
hundredth  part  of  its  value. 

The  discrepancies,  however,  which  were  plainly 
exhibited  in  the  motions  of  the  comet,  were  such  as 
to  excite  a  strong  suspicion  of  its  inaccuracy ;  and 
accordingly  a  new  determination  of  this  important 
quantity  was  attempted.  The  result  was  that  it  was 
found  by  three  different  computers,  each  by  an  en- 
tirely different  process,  that  the  mass  of  Jupiter, 
hitherto  assumed,  was  in  error  more  than  four  times 


ENCKE'S   COMET.  223 

the  amount  which  Laplace  had  assigned  as  the  limits 
of  its  probable  uncertainty.  "With  this  new  value 
of  the  mass  of  Jupiter,  and  admitting  the  existence 
of  a  resisting  medium,  it  seemed  possible  to  an- 
nounce the  future  movements  of  the  comet  with 
great  precision.  The  next  return  was  therefore  pre- 
dicted to  take  place  in  1842,  and  on  the  8th  of 
February  of  that  year  the  comet  was  detected  by 
Galle,  at  Berlin.  It  was  observed  in  Europe  and  in 
the  United  States  until  near  the  middle  of  April, 
and  was  found  to  follow  the  computed  path  very 
closely.  In  1848  it  was  observed  for  a  period  of 
three  months,  extending  from  August  to  November. 
When  first  detected  it  was  very  faint,  but  towards 
November  it  had  increased  in  brilliancy  so  as  to  be 
barely  perceptible  to  the  naked  eye  as  a  dull  hazy 
star  of  the  sixth  magnitude.  The  observations 
made  at  this  return  of  the  comet  have  not  only  con- 
firmed the  supposed  existence  of  an  ethereal  medium 
of  resistance,  but  have  also  been  the  means  of  de- 
riving some  very  important  results  in  relation  to  the 
changes  which  its  magnitude  or  volume  underwent 
in  approaching  the  sun,  and  also  in  receding  from  it. 
The  next  return  of  the  comet,  which  has  received 
the  name  of  Enckes  Comet,  in  honor  of  the  disco- 
verer of  its  periodic  character,  took  place  in  1852. 
It  passed  its  perihelion  on  the  14th  of  March  of  that 
year,  but  was  first  seen  on  the  2d  of  January.  The 


224  TREATISE   ON   COMETS. 

next  return  occurred  in  1855 ;  but  the  comet  was 
not  favorably  situated  for  observation  in  the  north- 
ern hemisphere.  It  was  observed  at  the  Cape  of 
Good  Hope,  and  also  in  Santiago,  in  Chili.  The 
comet  again  appeared  in  1858,  almost  exactly  in  the 
place  assigned  by  computation.  It  came  to  its 
perihelion  on  the  18th  of  October  of  that  year ;  but 
was  first  observed,  at  Berlin,  on  the  7th  of  August. 
From  the  observations  made  at  the  Cape  of  Good 
Hope  in  1855,  in  combination  with  those  made  pre- 
viously, at  the  successive  returns  of  the  comet,  Pro- 
fessor Encke  determined  its  orbit  anew ;  and  having 
made  due  allowance  for  the  planetary  perturbations, 
he  constructed  an  ephemeris,  and  on  the  first  clear 
evening  after  the  comet  was  computed  to  be  in  a 
position  for  observation,  the  telescope  was  directed 
to  the  spot  indicated,  and  the  comet  was  barely  dis- 
tinguishable as  a  dim  nebulosity,  without  any  dis- 
tinct or  regular  outline.  In  a  few  weeks,  however, 
the  brilliancy  had  increased  to  such  an  extent,  that 
when  seen  through  a  telescope,  the  comet  appeared 
as  a  round,  well-defined  nebula,  without  a  tail,  but 
having  decided  indications  of  a  point  near  the  centre, 
in  which  the  light  was  strongly  condensed.  About 
the  middle  of  September  it  became  visible  to  the 
naked  eye,  but  only  as  a  star  of  the  sixth  or  seventh 
magnitude,  shining  through  a  thin  haze.  At  this 
time  the  comet  was  seen  in  the  east  several  hours 


E  N  c  K  E  '  s   COMET.  225 

before  sunrise ;  and  being  in  a  position  very  favor- 
able for  observation,  it  was  possible  to  detect  a  very 
slight  elongation  of  the  head  of  the  comet  in  a 
direction  opposite  the  sun.  The  nucleus  was  almost 
a  stellar  point,  with  a  coma  or  envelope  of  very 
dense  nebulous  matter.  The  comet  was  not  ob- 
served after  the  first  week  in  October,  having  been 
lost  in  the  approaching  twilight.  The  next  return 
of  the  comet  will  take  place  in  March,  1862. 

The  orbit  of  Encke's  comet  is  an  ellipse  of  great 
eccentricity,  whose  length  is  about  double  its 
breadth.  When  at  its  perihelion  it  is  distant  from 
the  sun  only  32,000,000  miles,  or  a  little  less  than 
the  distance  of  Mercury.  When  in  the  aphelion  of 
its  orbit  the  comet  is  distant  from  the  sun  about 
400,000,000  miles,  or  a  little  more  than  four  times 
the  mean  distance  of  the  earth.  One  extremity  of 
the  orbit,  therefore,  reaches  a  little  beyond  the  orbit 
of  the  asteroid  Pallas,  and  the  other  extends  to  that 
of  Mercury.  The  length  of  the  orbit,  however,  is 
found  to  be  gradually  decreasing;  and,  conse- 
quently, the  period  of  revolution  is  becoming  shorter 
and  shorter.  This  phenomenon  has  been  attributed 
to  the  existence  of  a  resisting  medium,  consisting 
of  a  subtle  ethereal  fluid,  pervading  all  space.  The 
decrease  of  the  period  of  revolution  of  the  comet, 
after  making  due  allowance  for  the  perturbations 
whish  have  been  caused  by  the  action  of  the  planets, 


226 


TREATISE   ON   COMETS. 


is  exhibited  in  the  following  table,  from  1786,  the 
date  of  its  first  appearance,  till  1858,  the  time  of  its 
last  return  to  its  perihelion : 


Date. 

Period  of  revolu- 

tion in  days. 

1789, 

....   '1212.79 

1792, 

....     1212.67 

1795, 

....     1212.55 

1799, 

....     1212.44 

1802, 

....     1212.33 

1805, 

I*:-;,    .     .     1212.22 

1809, 

.    ...    .     .     1212.10 

1812 

.     .     .     .     1212.00 

1815, 

....     1211.89 

1819 

....     1211.78 

1822, 

....     1211.66 

Date. 


Period  of  revolu- 
tion in  days. 

1825,  ....  1211.55 

1829,  ....  1211.44 

1832,  ....  1211.32 

1835,  ....  1211.22 

1838,  ....  1211.11 

1842,  ....  1210.98 

1845,  ....  1210.88 

1848,  ....  1210.77 

1852,  ....  1210.65 

1855,  ....  1210.55 

1858,  1210.44 


The  existence  of  some  retarding  force  is  here  very 
strongly  exhibited.  The  action  of  the  planets  has 
been  computed  with  the  greatest  possible  precision  ; 
and  yet  the  period  is  decreasing,  by  slow  and  regu- 
lar degrees,  at  the  rate  of  about  eleven  hundredths 
of  a  day,  or  about  two  hours  and  thirty-eight  min- 
utes, at  each  successive  revolution.  This  is  evi- 
dently the  result  which  would  be  produced  by  a 
resisting  medium,  such  as  is  supposed  to  exist,  since 
the  effect  of  the  resistance  is  to  diminish  the  velocity 
of  the  comet  in  its  orbit,  thus  lessening  the  force 
resulting  from  the  orbital  motion,  or  what  is  called 
the  centrifugal  force,  and  consequently  compelling 
the  comet  to  fall  towards  the  sun,  in  obedience  to 
the  now  preponderant  force  of  gravity,  until  the 


ENCKE'S   COMET.  227 

equilibrium  is  again  restored.  The  comet  will  there- 
fore be  continually  describing  a  new  orbit, —  smaller 
than  the  one  in  which  it  had  previously  moved, — 
and  with  an  increased  velocity.  Thus  it  may  be 
perceived  that  the  medium  of  resistance  actually 
accelerates  the  motions  of  the  comet ;  but  since  the 
force  of  resistance  is  confined  to  the  plane  of  the 
orbit,  it  can  have  no  influence  whatever  in  producing 
any  additional  changes  in  the  elements.  In  com- 
puting the  effect  of  the  resisting  medium,  it  is 
assumed  to  increase  inversely  as  the  squares  of  the 
distances,  and  that  its  resistance  acts  as  a  force  tan- 
gent to  the  orbit,  and  proportional  to  the  squares 
of  the  comet's  actual  velocity  in  each  point  of  its 
orbit.  We  shall  subsequently  give  a  complete  ex- 
planation of  the  theory  of  this  ethereal  medium, 
and  also  consider  its  ultimate  effect  on  the  motions 
of  the  heavenly  bodies,  and  more  especially  on  those 
of  the  comets. 

This  comet,  as  we  have  already  stated,  is  a  small 
telescopic  object,  of  extreme  tenuity,  very  pale  and 
diffused  in  its  appearance,  even  at  the  point  of 
greatest  condensation,  and  without  a  tail.  It  has, 
however,  although  apparently  an  insignificant  ob- 
ject, opened  a  wider  field  of  discovery,  and  more 
new  and  interesting  facts,  than  any  other  on  record. 
It  has,  as  above  noticed,  furnished  the  means  of 
proving  the  existence  of  an  ethereal  fluid,  pervading 


228  TREATISE   ON    COMETS. 

all  space ;  and  has  afforded  a  striking  instance  of 
the  changes  which  take  place  in  the  nebulous  matter 
on  approaching  the  sun,  and  in  retreating  from  him ; 
and  also,  an  opportunity  of  measuring  these  changes. 
Stars  which  have  been  seen  through  it  more  than 
once,  have  attested  the  high  degree  of  its  attenua- 
tion ;  while  the  smallness  of  its  orbit,  and  the  short- 
ness of  its  period,  give  it  almost  the  characteristics 
of  a  planet.  The  constant  decrease  in  the  size  of 
its  orbit,  arising  from  the  resistance  of  the  ethereal 
fluid,  may  possibly  precipitate  it  upon  the  surface 
of  the  sun,  if,  before  that  event  can  take  place,  the 
perpetual  diminution  of  its  mass  does  not  offer  an 
example  of  condensation  to  the  solid  state,  or  of 
annihilation ;  and  its  perturbation  by  the  planets 
have  been  the  means  of  indicating  errors  in  the 
assumed  masses  of  Mercury  and  Jupiter. 

At  each  successive  return,  this  comet  furnishes 
additional  evidence  in  support  of  the  hypothesis  of 
a  resisting  medium.  Indeed,  it  has  already  incon- 
trovertibly  established  the  fact  of  the  existence  of 
such  a  medium,  which,  if  there  be  not  other  consi- 
derations to  counterbalance,  may  result  ultimately, 
though  at  a  time  almost  infinitely  distant,  in  the 
total  annihilation  of  the  planetary  system.  It  is  a 
result  so  startling  in  its  consequences,  and  so  repug- 
nant to  our  preconceived  ideas  of  the  stability  of 
our  system,  that  the  mind  almost  shrinks  in  horror 


BIELA'S   COMET.  229 

from  its  contemplation.  If  it  were  not  that  the  cal- 
culations from  which  such  a  conclusion  has  been 
derived,  have  heen  repeated  by  different  individuals, 
each  one  using  different  methods  of  computation, — 
but  all  arriving  at  the  same  general  results,  —  we 
might  perhaps  hesitate  to  receive  it.  But  when  evi- 
dence of  the  most  unequivocal  character  has  been 
adduced,  we  are  compelled  to  admit  the  fact,  and 
look  to  other  forces  to  prevent  the  inevitable  conse- 
quences which  must  ensue.  There  can  be  but  little 
doubt  that  there  are  other  forces  in  nature  than 
those  now  known,  which  time  alone  will  unfold;  and 
which  will  enable  the  astronomers  of  some  future 
day  to  explain  all  these  mysterious  questions,  to 
dispel  all  such  dismal  forebodings  as  the  existence 
of  a  resisting  medium  in  space,  with  our  present 
knowledge,  will  excite ;  and  to  furnish  finally  the 
most  sublime  proofs  of  the  fixed  design  of  perpe- 
tuity which  the  Almighty  Creator  has  enstamped  on 
his  universe  of  creation.  Let  us,  therefore,  be  con- 
tent with  our  present  knowledge  of  laws  and  forces ; 
and,  if  it  be  not  permitted  to  our  day  and  genera- 
tion, leave  it  to  those  who  shall  come  after  us  to 
solve  these  difficult  problems. 

The  discovery  of  this  comet's  periodic  character  was 

soon  followed  by  the  announcement  of  a  second  comet 

of  short  period.     This  comet  was  detected  by  Biela, 

an  Austrian  officer,  at  Josephstadt,  in  Bohemia,  on 

20 


230  TREATISE   ON   COMET?. 

the  27th  of  February,  1826 ;  and  was  subsequently 
independently  discovered  by  Gambart,  at  Marseilles, 
on  the  9th  of  March  of  the  same  year.  It  was  a 
faint,  nebulous  object,  not  unlike  Encke's  comet  in 
general  appearance,  and  was  observed  only  about 
two  months.  As  soon,  however,  as  the  elements  of 
its  orbit  had  been  computed,  it  was  found  that  the 
comet  was  probably  identical  with  one  which  had 
appeared  in  1805,  and  for  which  Gauss  had  obtained 
an  elliptic  orbit,  corresponding  to  a  period  of  about 
six  years.  A  more  complete  investigation  of  the 
orbit  showed  that  it  arrived  at  its  perihelion  a  little 
before  midnight,  at  Paris,  on  the  18th  of  March, 
1826 ;  and  that  the  orbit  in  which  it  was  then  moving 
was  an  ellipse,  giving  a  period  of  revolution  of  2460 
days,  or  about  six  years  and  nine  months.  It  was 
also  found,  in  tracing  its  path  backwards,  that  the 
comet  was  observed  in  1772  and  in  1805 ;  and  that, 
between  the  former  date  and  1826,  it  had  performed 
eight  revolutions.  It  was  therefore  predicted  that  it 
would  return  again  in  1832 ;  and  on  the  26th  of 
November,  of  that  year,  it  passed  its  perihelion  in 
almost  exact  accordance  with  the  prediction.  It  was 
observed  during  the  months  of  October,  November, 
and  December ;  and  was  last  seen,  at  the  Cape  of 
Good  Hope,  on  the  3d  of  January,  1833.  At  this 
return  it  was  very  faint,  and  was  observed  with  the 
greatest  difficulty. 


BIE LA'S   COMET.  231 

The  orbit  of  this  comet  is  an  ellipse  whose  eccen- 
tricity is  a  little  less  than  that  of  Eucke's  comet,  or 
about  0.75.    Its  perihelion  distance  is  nearly  84,000,- 
000  miles,   and   its   aphelion  distance   581,000,000 
miles.     It  therefore  comes  within  the  orbit  of  the 
earth,  and  recedes  beyond  the  orbit  of  Jupiter;  and 
since  the  plane  of  its  orbit  is  inclined  to  the  plane 
of  the  ecliptic  by  a  small  angle,  it  is  evident  that 
the  comet  may  approach  very  near  these  planets, 
and  thereby  be  subject  to  considerable  disturbance 
in  its  motions.     Moreover,  it  happens  that  one  of 
the  points  in  which  Biela's  comet  intersects  the 
plane  of  the  ecliptic,  is  at  a  distance  from  the  earth's 
orbit  less  than  the  sum  of  the  semi-diameters  of  the 
earth  and  comet.     It  follows,  therefore,  that  in  case 
the  earth  should  ever  arrive  at  this  point  of  its  orbit 
at  the  same  moment  at  which  the  comet  passes 
through  the  point  of  its  orbit  which  is  nearest  to  it, 
a  portion  of  the  head  of  the  latter  would  envelop 
the  earth.     It  was,  indeed,  found,  on  the  occasion 
of  the  return  of  this  comet  in  1832,  that  if,  by  any 
unforeseen  circumstance,  the  comet  should  be  de- 
layed one  month,  it  would  come  in  contact  with  the 
earth  on  the  30th  of  November.     This  announce- 
ment created  a  profound  sensation,  and  no  small 
degree  of  alarm  among  those  unacquainted  with  the 
character  and  motions  of  such  bodies.     But  these 
apprehensions  were  wholly  unfounded  —  the  comet 


232  TREATISE   ON    COMETS. 

having  passed  that  point  of  its  orbit  on  the  29th  of 
October,  at  a  distance  from  the  earth  of  more  than 
60,000,000  miles. 

At  its  return  in  1839,  the  comet  was  most  unfa- 
vorably situated  for  observation,  being  in  the  imme- 
diate vicinity  of  the  sun ;  and  consequently  is  not 
known  to  have  been  seen  from  any  part  of  the  globe. 
The  next  return,  however,  occurred  under  very 
favorable  circumstances.  From  the  observations 
made  at  its  previous  returns  to  its  perihelion,  San- 
tini  determined  the  elements  of  its  orbit  with  great 
care ;  and  having  computed  the  perturbations  of 
the  planets  during  the  period  in  which  it  was  invisi- 
ble between  1832  and  1845,  he  computed  an  ephem- 
eris,  by  means  of  which  it  was  discovered  by  De 
Yico,  at  Rome,  on  the  26th  of  November,  1845.  It 
was  seen  at  Berlin  on  the  28th  of  November;  at 
Cambridge,  England,  on  the  1st  of  December ;  and 
subsequently  at  all  the  principal  observatories  in 
Europe.  It  continued  to  be  visible  until  the  end  of 
April,  1846,  having  passed  its  perihelion  on  the  llth 
of  February  of  that  year. 

This  comet,  as  already  stated,  is  a  small,  round, 
nebulous  object,  not  visible  to  the  naked  eye  even 
when  brightest,  and  without  a  tail.  It  presents  no 
solid,  or  even  well-defined  nucleus ;  and  appears  to 
be  a  mere  mass  of  vapor,  so  extremely  attenuated 
that  minute  stars  can  be  seen  through  it,  although 


B  i  EL  A  's   COMET.  233 

its  diameter  probably  exceeds  50,000  miles.  The 
deficiency,  however,  in  brilliancy  and  magnitude,  is 
fully  compensated  by  the  anomalous  phenomena 
which  it  presents,  of  two  distinct  comets,  each  com- 
plete in  itself,  and  revolving  around  the  sun  in  such 
close  proximity  as  to  be  almost  always  visible  simul- 
taneously in  the  field  of  the  telescope.  In  1832, 
and  at  its  previous  returns,  it  appeared  as  a  single 
body,  with  a  point  of  condensed  light  near  its  cen- 
tre; and  when  first  observed  in  1845,  there  was 
nothing  unusual  noticed  in  its  appearance.  About 
the  middle  of  December,  1845,  it  was  noticed  that 
the  comet  gave  positive  indications  that  some  vio- 
lent change  of  its  form  was  about  to  be  effected. 
On  the  19th  of  December,  it  is  described  as  being 
pear-shaped ;  the  nebulosity  being  unduly  elongated 
in  a  direction  different  from  that  in  which  a  tail 
would  be  seen.  On  the  29th  of  December,  it  was 
seen  attended  by  a  faint,  nebulous  spot,  which  gave 
it  the  appearance  of  having  a  short  tail  connected 
with  the  envelope  of  the  nucleus  by  a  denser  mass 
of  nebulous  matter.  But  on  the  13th  of  January  it 
was  noticed  at  Washington,  that  instead  of  being, 
as  usual,  a  single  comet,  it  apparently  consisted  of 
two  comets,  moving  through  space  side  by  side. 
Each  body  is  said  to  have  exhibited  all  the  charac- 
teristics of  a  telescopic  comet,  being  gradually  con- 
densed towards  the  centre,  without  any  distinct  and 
20* 


284  T  u  i;  A  T  i  s  E   ON    COMETS. 

well-defined  disc ;  and  each  elongated  on  the  side 
opposite  the  sun.  They  were,  however,  of  unequal 
size ;  one  being  at  least  eight  times  larger  than  the 
other,  and  about  four  times  brighter. 

The  separation  of  the  comet  into  two  distinct 
parts,  was  observed  at  several  places  in  Europe,  on 
the  15th  of  January,  1846,  and,  subsequently,  at  all 
the  principal  observatories.  The  smaller  comet  was 
supposed  to  have  been  thrown  off  from  the  nucleus 
of  the  larger  one,  which  was  regarded  as  the  original 
comet.  The  latter  was,  as  above  noticed,  at  first  a 
globular  mass  of  nebulous  matter,  semi-transparent 
at  its  very  centre,  and  without  any  tail ;  while,  after 
the  separation,  both  comets  showed  decided  indica- 
tions of  short  trains,  parallel  in  their  direction,  and 
at  right  angles  to  the  line  joining  the  centres  of  the 
nuclei.  Again,  it  was  observed  that  from  the  day 
of  their  separation,  the  original  comet  decreased 
both  in  magnitude  and  brilliancy,  and  the  companion 
increased  until  near  the  middle  of  February,  when 
they  were  sensibly  equal.  After  this  the  companion 
still  increased,  and  from  the  14th  to  the  16th  of 
February,  was  not  only  much  brighter  than  the 
original  comet,  but  had  a  distinct  stellar  nucleus. 
The  change  of  brilliancy  was  again  reversed,  the 
original  comet  recovering  its  superiority  in  this  re- 
spect, and  acquiring,  on  the  18th  of  February,  the 
same  appearance  which  was  presented  by  the  com- 


BIELA'S   COMET.  235 

panioii  from  the  14th  to  the  16th  of  the  same  month. 
After  this  the  companion  gradually  faded  away,  and 
on  the  1st  of  March  could  only  be  seen  with  great  • 
difficulty.  It  was  last  observed  on  the  15th  of 
March,  and  subsequently  the  comet  again  appeared 
to  be  single. 

While  these  anomalous  changes  were  taking 
place,  there  were  indications  of  a  connection  between 
the  two  nuclei,  a  faint  arc  of  light  being  noticed, 
which  extended  as  a  kind  of  bridge  from  the  one  to 
the  other.  This  line  of  light,  however,  did  not 
occupy  any  fixed  position,  but  was  observed  to  have 
at  times  a  slight  angular  motion,  sometimes  in  one 
direction  and  sometimes  in  the  opposite  direction, 
and  thus  the  comets  were  seen  to  oscillate  about 
each  other,  according  to  some  mysterious  law  which 
remains  yet  to  be  fully  revealed. 

The  opposite  diagram  represents  the  appearance 
of  the  comet  as  seen  through  a  telescope  about  the 
middle  of  February,  1846. 

The  orbits  of  both  the  original  comet  and  its  com- 
panion, have  been  computed  as  if  they  were  separate 
and  independent  bodies,  by  Plantamour,  of  Genoa ; 
and  he  finds  that  the  real  distance  between  their 
centres  was,  during  the  greater  portion  of  the  period 
of  their  visibility,  subject  only  to  very  slight  varia- 
tions, being  about  150,000  miles.  The  same  com- 
putations were  performed  by  Hubbard,  of  the  !NTa- 


236 


TREATISE   UN    COMETS. 


tional  Observatory,  at  Washington,  who  finds  that 
the  entire  series  of  observations  may  be  satisfactorily 
represented  by  supposing  each  nucleus  to  describe 
an  independent  ellipse  around  the  sun.  It  is  found 
also  that  the  comets  moved  on  side  by  side,  without 
manifesting  any  reciprocal  disturbing  action ;  or 
that,  at  least,  if  any  such  disturbance  did  exist,  it 
was  too  minute  to  be  indicated  by  the  observations 
— a  circumstance  which  shows  conclusively  that  the 
masses  of  such  bodies  are  almost  infinitely  small. 
The  following  table  gives  the  distance  of  one  nucleus 
from  the  other,  as  determined  by  means  of  Hub- 
bard's  elements  of  the  orbits  : 


Distance,  in  miles. 
17,       .      .   226,000 

25,      .     .  216,000 

5,      .     .  202,000 

21,     .     .  170,000 


The  next  return  of  Biela's  comet  was  predicted 
to  take  place  in  1852 ;  and  on  the  morning  of  the 
26th  of  August,  of  that  year,  in  searching  for  the 
comet,  Professor  Secchi,  at  Rome,  detected  a  small 
nebulous  object,  which  was  very  far  from  the  position 
of  the  cornet  given  by  the  ephemeris.  It  was  soon 
found  that  the  faint  nebulosity  observed  was  really 
a  comet,  and  still  later  that  it  was  the  one  sought 
for.  "No  companion,  however,  was  visible ;  but  on 
the  morning  of  the  16th  of  September  following, 


Date. 

Distance,  in  miles. 

Date. 

1845,  Dec.     1, 

.    .    .  117,000 

1846,  Feb. 

1846,  Jan.  20, 

.     .     .  198,000 

«        « 

"     28, 

.     .     .  213,000 

"     Marc 

"     Feb.    9, 

.     .     .  227,000 

a        n 

BIELA'S   COMET. 


237 


this  was  also  detected.  It  was  very  faint,  and  of  an 
elongated  ovoid  form,  the  apex  being  turned  away 
from  the  sun.  It  was  distant  from  the  other  part — 
which  was  supposed  to  he  the  original  comet,  in 
1846— a  little  more  than  half  a  degree.  The  latter, 
at  the  same  time,  appeared  quite  irregular,  and  had 
two  very  faint  streaks  of  light  issuing  from  it,  but 
did  not  exhibit  a  distinct  nucleus,  although  more 
luminous  in  the  centre  than  at  the  edges.  The 
original  comet  was  last  seen  on  the  25th  of  Septem- 
ber, and  the  companion  on  the  28th  of  the  same 
month.  The  distances  of  the  two  comets  from  each 
other  were  as  follows : 


Date.  Distance,  in  miles. 

1852,  Sept.  16,  .     .   1,616,000 

"     20,  .     .   1,624,000 

"     24,  .     .   1,625,000 

"     28,  .     .   1,619,000 


It  is  evident  from  this  table  of  the  distances  of 
the  nuclei  from  each  other,  that  in  1852  the  two 
comets  moved  nearly  as  in  1846,  with  the  exception 
only  that,  at  this  return,  the  distance  of  the  two 
bodies  was  nearly  seven  times  greater  than  in  1846. 
We  might  expect,  therefore,  that  it  would  be  easy 
to  determine  the  orbit  of  each  nucleus  with  great 
precision,  from  the  observations  made  in  1846  and 
in  1852.  But  it  is  found,  unfortunately,  that  it  is 


Date. 

Distance,  in  miles. 

1852,  Aug.  27, 

.     .   1,501,000 

"     31, 

.     .    1,531,000 

"     Sept.    4, 

.     .    1,559,000 

8, 

.     .   1,582,000 

"     12, 

.     .   1,601,000 

238  TREATISE   ON   COMETS. 

nearly  impossible  to  decide  which  of  the  two  comets 
observed  in  1852  is  the  original  comet  of  1846,  since 
all  the  observations  are  represented  almost  equally 
well,  by  either  the  hypothesis  that  the  one  disco- 
vered by  Secchi,  on  the  25th  of  August,  was  the 
original  comet  of  1846,  or  that  the  one  found  on  the 
16th  of  September  is  the  identical  one.  The  first 
supposition  has  been  most  generally  adopted,  and 
the  time  of  the  next  return  of  the  comet,  in  1859, 
was  computed  in  accordance  with  this  assumption. 
It  was  predicted  that  the  original  comet  would 
arrive  at  its  perihelion  on  the  24th  of  May,  1859,  a 
little  before  midnight  at  Greenwich,  and  that  the 
companion  would  arrive  at  the  point  of  its  orbit 
nearest  the  sun  about  thirty-five  hours  earlier. 
They  were,  however,  situated  in  a  position  very 
unfavorable  for  observation,  being  in  the  immediate 
vicinity  of  the  sun ;  and  although  there  can  be  no 
doubt  but  that  they  returned  very  nearly  in  accord- 
ance with 'the  prediction,  yet  for  the  reasons  here 
stated,  no  observation  is  known  to  have  been  made. 
This  is  much  to  be  regretted,  since  a  few  observa- 
tions in  1859  would  have  definitely  decided  the 
question  of  identity  of  the  two  comets  at  the  pre- 
vious returns,  and  would  have  greatly  facilitated  all 
future  investigations  respecting  their  motions.  The 
comets  will  again  return  in  1866,  and  will  then  be 
in  a  position  favorable  for  observation;  and  we 


BIELA'S   COMET.  239 

must,  therefore,  await  this  appearance  in  order  to 
settle  the  important  question  of  identity. 

Such  are  the  most  prominent  characteristics  of 
the  phenomena  presented  by  Biela's  comet,  at  its 
successive  returns  to  its  perihelion ;  and  on  account 
of  the  anomalous  appearance  of  a  double  comet 
here  exhibited,  it  may  not  be  improper  to  introduce, 
in  this  connection,  a  statement  of  the  various  hypo- 
theses which  have  been  devised,  in  order  to  explain 
such  an  unexampled  departure  from  the  general 
form  of  such  bodies.  In  the  first  place,  then,  it  is 
found,  by  comparing  the  distances  of  the  comets 
from  each  other  in  1846  and  in  1852,  that  the  sepa- 
ration must  have  taken  place  about  500  days  before 
the  perihelion  passage  in  1846,  or  about  the  middle 
of  July,  1843.  This  date,  however,  is  still  quite 
uncertain;  and  it  will  not  be  possible,  until  this  has 
been  definitely  determined,  to  decide  under  what 
circumstances  the  separation  of  the  nucleus  oc- 
curred, and  by  what  means  it  was  effected  in  such 
a  way  as  not  to  change  or  influence  the  motions  of 
the  primary  nucleus.  Again,  the  extraordinary 
changes  in  brilliancy  which  the  two  comets  exhi- 
bited, both  in  1846  and  1852,  clearly  indicate  that 
the  brightness  of  these  objects  does  not  depend 
merely  on  their  distance  from  the  earth  and  sun, 
but  upon  other  unknown  causes.  These  causes 
may,  indeed,  have  been  sufficient  to  develop  the 


240  TREATISE   ON   COMETS. 

brilliancy  of  the  companion  of  Biela's  comet,  at  its 
last  two  returns  to  the  sun,  to  such  an  extent  as  to 
render  it  visible  from  the  earth ;  while,  at  its  former 
returns,  on  account  of  its  unfavorable  position,  and 
the  inefficient  operation  of  these  causes,  it  might 
have  been  too  faint  to  be  noticed.  The  great  change, 
however,  in  the  distances  of  the  nuclei  from  each 
other  from  1846  to  1852,  seems  to  be  sufficient  in 
itself  to  show  that  the  separation  did  not  take  place 
until  after  the  return  of  the  comet  in  1832. 

The  most  plausible  view  of  the  cause  of  this  per- 
manent separation  of  a  comet  into  two  distinct  parts, 
is  that  which  assumes  a  repulsive  force  emanating 
from  the  sun,  the  nature  and  effect  of  which  we 
shall  fully  consider  when  we  come  to  treat  of  the 
theory  of  the  physical  constitution  of  comets,  and 
of  the  formation  of  their  tails ;  it  being  only  neces- 
sary to  state  here,  that  the  phenomena  presented  by 
Halley's  comet  in  1835,  and  by  the  great  comet  of 
1858,  have  demonstrated  the  existence  of  such  a 
force.  The  effect  of  a  repulsive  force  upon  the 
nebulous  envelope  of  the  comet,  would  be  to  distort 
it  from  the  spherical  form, — which  it  would  assume 
if  the  force  of  gravity  alone  acted,  —  causing  it  to 
become  an  ellipse,  the  length  of  which,  compared 
with  its  breadth,  would  be  greater  as  the  repulsive 
force  increases.  The  repulsive  force  may  now  be 
conceived  to  become  so  great  as  to  drive  the  remoter 


F  A  y  E  '  s   COMET. 

particles  of  the  envelope  beyond  the  influence  of 
the  nucleus,  and  carry  them  off  into  space.  The 
entire  mass  of  the  comet  will  therefore  be  urged 
toward  the  sun  by  the  difference  of  the  total  attract- 
ive and  repulsive  forces.  So  long  as  the  repulsive 
is  insufficient  to  separate  the  matter  composing  the 
comet,  it  will  revolve  as  a  single  body,  elongated  in 
a  direction  opposite  the  sun;  but  if  the  repulsive 
force  should  be  so  great  as  to  overcome  the  gravi- 
tation of  the  nucleus,  the  exterior  portions  of  the 
envelope  will  be  successively  driven  off  into  space, 
until  finally  a  new  comet  is  formed,  which  will 
henceforth  revolve  as  a  companion  to  the  original 
one,  as  in  the  case  of  Biela's  comet. 

The  successive  returns  of  this  comet  have  also 
indicated  the  existence  of  a  resisting  medium,  the 
period  being  always  less  than  at  the  preceding  peri- 
helion passage.  In  1852,  it  came  to  the  perihelion 
nearly  two  days  in  advance  of  the  predicted  time, 
after  due  allowance  had  been  made  for  the  action 
of  the  planets  on  the  comet  during  the  entire  inter- 
val elapsed  since  1846. 

On  the  22d  of  November,  1843,  a  telescopic  comet 
was  discovered  by  M.  Faye,  of  the  Royal  Observa- 
tory, at  Paris,  which  was  subsequently  found  to 
move  in  an  elliptic  orbit,  with  a  period  of  revolution 
of  about  seven  and  a  half  years.  When  first  disco- 
vered, the  comet  had  a  brilliant  nude/us,  and  a  short 
21  Q 


242f  TREATISE   ON    COMETS. 

fan-shaped  tail,  about  one-tenth  of  a  degree  in 
length.  It  was  last  seen  at  Pulkova,  in  Russia,  on 
the  10th  of  April,  1844.  It  passed  its  perihelion  on 
the  17th  of  October,  1844,  at  a  distance  from  the 
sun  of  161,000,000  miles.  Its  aphelion  distance  is 
563,000,000  miles.  The  comet,  therefore,  does  not 
come  within  the  orbit  of  the  planet  Mars,  and  re- 
cedes, in  its  aphelion,  beyond  the  orbit  of  Jupiter. 
But  the  most  remarkable  feature  connected  with  the 
orbit  of  this  cornet,  is  the  eccentricity,  which  is  only 
a  little  more  than  that  of  the  orbits  of  some  of  the 
asteroid  planets,  thus  forming  a  connecting  link 
between  the  cometary  and  planetary  systems. 

As  soon  as  the  entire  series  of  observations  of  the 
comet,  made  in  1843  and  1844,  had  been  published, 
a  complete  investigation  of  the  elements  of  its  orbit 
was  accomplished;  and  the  result  was,  that  its  period 
of  revolution  was  found  to  be  2718  days,  or  a  little 
less  than  seven  and  a  half  years.  It  was  also  no- 
ticed that  the  position  of  its  orbit  is  such  that  the 
comet  is  liable  to  excessive  perturbations,  on  account 
of  its  close  proximity,  when  near  the  aphelion,  to 
the  orbit  of  Jupiter.  In  1840  it  was  so  near  this 
planet,  that  his  attraction  was  about  one-tenth  part 
of  that  of  the  sun,  which  must  necessarily  have 
caused  a  great  alteration  of  the  form  of  its  orbit. 
In  1815,  also,  it  probably  came  still  nearer  to  Jupiter, 
and  the  perturbations  must  have  been  much  greater 


F  AYE'  s   COME  T.  243 

than  in  1840.  Under  such  circumstances,  M.  Yalz 
was  led  to  conclude  that,  prior  to  1840,  the  comet 
moved  in  an  orbit  entirely  different  from  that  in 
which  it  is  now  moving;  and  that,  possibly,  this 
comet  might  be  identical  with  a  comet  which  ap- 
peared in  1770,  and  which,  although  at  that  time 
moving  in  an  ellipse  of  only  five  years,  has,  by  the 
action  of  Jupiter,  been  so  excessively  disturbed  in 
its  motions  that  it  has  not  since  been  seen. 

A  complete  investigation  of  this  problem  was 
subsequently  undertaken  by  Le  Yerrier,  who  finds, 
after  computing  the  perturbations  of  Faye's  comet, 
by  Jupiter,  that  it  cannot  be  identical  with  the 
comet  of  1770.  He  also  computed  the  perturbations 
of  the  comet  arising  from  the  attraction  of  the 
planets  during  the  interval  from  1843  to  1851,  and 
predicted  that  it  would  return  to  its  perihelion  on 
the  3d  of  April,  1851.  With  the  aid  of  Le  Yerrier  s 
elements,  an  ephemeris  was  computed  for  the  latter 
part  of  1850,  and  beginning  of  1851,  by  means  of 
which  observers  might  be  enabled  more  easily  to 
detect  the  comet  at  its  return  ;  and  more  especially 
because  it  was  foreseen  that  it  would  be  extremely 
faint  and  small,  and  not  capable  of  being  seen  ex- 
cept through  the  most  powerful  telescopes.  By 
means  of  this  ephemeris  the  comet  was  re-discovered 
at  the  observatory  at  Cambridge,  England,  on  the 
25th  of  December,  1850,  and  was  followed  until  the 


244  TKEATISE   ON    COMETS. 

4th  of  March,  1851.  It  was  subsequently  observed 
at  Cambridge,  Mass.,  and  at  Pulkova,  in  Russia;  and 
the  positions  actually  observed  scarcely  differed  from 
those  predicted  by  means  of  Le  Verrier's  elements. 
The  comet  was  described  as  an  extremely  faint  object: 
so  much  so  as  to  be  barely  visible  through  the  large 
telescopes  of  those  observatories.  It  appeared  to  be 
slightly  elongated  in  the  direction  of  the  sun,  but  no 
well-defined  nucleus  or  point  of  condensation  could 
be  perceived.  It  came  to  its  perihelion  on  the  morning 
of  the  2d  of  April,  1851,  or  within  one  day  of  the 
predicted  time.  This  difference  is  undoubtedly  due, 
to  a  certain  extent,  to  the  influence  of  the  resisting 
medium. 

The  next  return  of  this  comet  took  place  in  1858. 
It  was  observed  during  the  months  of  September 
and  October,  and  passed  its  perihelion  on  the  12th 
of  the  former  month.  After  making  due  allowance 
for  the  action  of  the  planets  from  1851  to  1858,  it  is 
found  that  Le  Verrier's  elements  still  represent  the 
motion  of  the  comet  with  great  precision.  At  its 
last  return,  in  1858,  the  comet  was  very  faint  and 
ill-defined,  and  without  either  a  nucleus  or  a  tail, 
being,  apparently,  a  diffused  mass  of  nebulous 
matter.  The  next  return  to  the  perihelion  will  occur 
in  March,  1866. 

A  fourth  comet  of  short  period  was  discovered  by 
De  Vico,  at  Rome,  on  the  22d  of  August,  1844, 


©IS EAT  (D(MIET   (DF 


DE  Vice's   COMET.  245 

which,  in  the  moonlight,  was  almost  visible  to  the 
naked  eye.  It  was  also  discovered  independently 
by  Encke,  at  Berlin,  on  the  5th  of  September,  and 
on  the  following  evening  it  was  seen  at  Hamburgh, 
by  M.  Melhop,  an  amateur  astronomer.  In  America 
it  was  first  seen  by  Mr.  H.  L.  Smith,  at  Cleveland, 
Ohio,  on  the  10th  of  September.  Soon  after  the 
middle  of  this  month  it  became  distinctly  visible  to 
the  naked  eye,  and  when  seen  through  a  telescope, 
the  nucleus  presented  a  very  beautiful  appearance, 
being  bright  and  strongly  condensed.  The  tail  was 
about  one  degree  in  length,  extending  in  a  direction 
opposite  to  the  sun.  It  subsequently  decreased 
rapidly  in  brilliancy,  and  was  observed  for  the  last 
time  at  Pulkova,  on  the  31st  of  December.  The 
orbit  was  computed,  as  usual,  on  the  supposition  that 
it  was  a  parabola — since  this  is  more  easily  computed 
than  an  elliptic  orbit — but  it  was  soon  found  that  it 
was  impossible  to  represent  the  observations  by  such 
an  orbit,  and  accordingly  an  ellipse  was  computed, 
which  gave  a  periodic  time  of  about  five  and  a  half 
years.  It  passed  its  perihelion  on  the  2d  of  Sep- 
tember, 1844,  at  a  distance  from  the  sun  of  about 
114,000,000  miles.  When  in  the  aphelion  of  its 
orbit,  its  distance  from  the  sun  is  475,000,000  miles. 
The  inclination  of  the  plane  of  its  orbit  to  the  plane 
of  the  ecliptic,  is  less  than  three  degrees ;  and,  con- 
sequently, it  may  come  so  near  the  planet  Jupiter 
21* 


246  TREATISE   ON    COMETS. 

that  its  orbit  will  eventually  be  entirely  changed  by 
the  perturbations  caused  by  this  planet.  It  also 
comes  very  near  the  earth,  and  is  considerably  dis- 
turbed by  its  action. 

The  interest  which  the  appearance  of  the  comet, 
in  connection  with  its  short  period,  it  being  less  than 
that  of  any  other  which  had  hitherto  been  dis- 
covered, with  the  exception  of  Encke's  come.t,  in- 
duced the  Academy  of  Sciences,  at  Amsterdam,  to 
make  the  investigation  of  its  orbit  the  subject  of  a 
prize.  Accordingly,  a  thorough  determination  of 
the  elements  of  its  orbit  from  the  entire  series  of 
observations,  embracing  a  period  of  more  than  four 
months,  was  undertaken  by  Briinnow,  at  that  time 
assistant  in  the  Royal  Observatory  at  Berlin.  He 
computed  the  perturbations  of  the  comet  by  all  the 
planets  within  the  orbit  of  Uranus,  excepting,  of 
course,  the  asteroid  planets  between  Mars  and  Jupi- 
ter, and  finally  obtained  an  orbit  which  satisfied  all 
the  observations  with  extreme  precision.  The  period 
of  revolution  was  thus  found  to  be  1996J  days,  or 
very  nearly  5|  years.  The  comet  would,  therefore, 
return  again  to  its  perihelion  about  the  middle  of 
February,  1850 ;  but  it  happened,  very  unfortunately, 
that  it  was  so  near  the  sun  during  the  entire  period 
in  which  it  remained  sufficiently  near  the  earth  to 
be  visible,  if  seen  under  more  favorable  circum- 


D  K  •  V i c o ' s   COMET.  247 

stances,  with  respect  to  its  apparent  position  in  the 
heavens,  that  it  could  not  he  observed. 

The  next  return  of  the  comet  to  its  perihelion  oc- 
curred in  the  summer  of  1855,  and  since  it  would 
then  he  in  a  position  favorable  for  observation,  its 
reappearance  was  contemplated  with  no  small  degree 
of  interest.  The  perturbations  were  computed  from 
1844  to  1855,  and  Dr.  Briinnow  announced  that  it 
would  be  in  its  perihelion  on  the  evening  of  the  6th 
of  August,  1855.  As  soon  as  the  comet  was  com- 
puted to  be  near  enough  to  be  visible  from  the  earth, 
it  was  sought  for  in  the  position  indicated  by  the 
ephemeris,  but  without  success.  The  search  was 
continued  until  the  comet  was  probably  out  of  reach 
of  the  most  powerful  instruments,  and  it  is  not  cer- 
tainly known  that  it  was  seen  at  any  part  of  the 
globe.  On  the  16th  of  May,  1855,  Goldschmidt,  of 
Paris,  while  searching  for  the  comet,  found  a  faint 
nebulous  body,  with  an  ill-defined  and  irregular  out- 
line, and  without  a  tail,  in  a  position  which  did  not 
differ  much  from  the  computed  place  of  De  Yico's 
comet,  if  it  be  assumed  that  the  perihelion  passage 
took  place  on  the  llth  of  August.  The  comet,  how- 
ever, must  have  been  much  nearer  the  earth  on  the 
1st  of  August  than  at  the  date  of  this  observation, 
while  its  position  in  the  heavens  must  have  been 
much  more  favorable  for  observation.  It  seems 
certain,  therefore,  that,  since  the  search  continued 


248  TREATISE   ON   COMETS. 

until  after  the  beginning  of  August,  this  nebulous 
object  could  not  have  been  the  comet;  and  we  are 
compelled  to  conclude  that  it  was  nowhere  seen  in 
1855 — a  failure  to  realize  the  predictions,  which  may 
be  accounted  for,  not  by  supposing  the  methods  of 
computation  to  have  been  inaccurate,  but  rather  by 
the  uncertainty  of  the  ephemeris,  owing  to  the  great 
difficulty  of  determining  the  time  of  perihelion 
passage  in  1855  from  the  observations  made  in  1844. 
The  comet  may  be  expected  to  return  again  toward 
the  latter  part  of  1860,  or  in  the  beginning  of  1861 ; 
but  it  will  not  probably  be  in  a  position  favorable 
for  observation.  This  is  to  be  regretted,  since  the 
comet,  before  it  has  accomplished  the  succeeding 
revolution,  will  be  subject  to  great  perturbations  by 
Jupiter,  which  may  so  completely  change  its  orbit, 
that  it  will  be  difficult  to  identify  it  at  any  future 
return. 

Another  important  circumstance  to  be  noticed  in 

connection  with  this  comet,  is  that  its  orbit  is  very 

•% 

similar  to  that  of  a  comet  which  appeared  in  1585. 
Some  astronomers  have,  for  this  reason,  supposed 
that  De  Yico's  comet  is  identical  with  the  comet  of 
1585 ;  and  possibly,  also,  with  those  of  1743,  1766, 
and  1819.  Le  Yerrier  and  Briinnow  have  under- 
taken to  decide  this  question  by  direct  computation ; 
and  they  conclude,  finally,  that  this  comet  cannot 
be  identical  with  either  of  those  just  mentioned,  nor 


BRORSEN'S   COMET.  249 

with  any  other  on  record,  unless  it  be  with  one 
which  appeared  in  1678.  They  find  that  there  is  a 
strong  probability  of  identity  in  this  case,  although 
the  perturbations  in  the  meantime  have  very  mate- 
rially changed  the  elements  of  its  orbit. 

Another  periodic  comet  was  discovered  by  Bror- 
sen,  at  Kiel,  in  Denmark,  on  the  26th  of  February, 
1846.  It  was  a  faint,  telescopic  object,  and  was 
found  to  move  in  an  elliptic  orbit,  with  a  periodic 
time  of  about  five  and  a  half  years ;  and  has  been 
supposed  to  be  identical  with  the  comets  of  1532 
and  1661.  It  was  observed  until  the  22d  of  April ; 
and  from  the  entire  series  of  observations,  its  exact 
period  was  ascertained  to  be  2039  days.  Its  peri- 
helion distance  is  about  62,000,000  miles.  When  in 
the  aphelion,  its  distance  from  the  sun  is  532,000,000 
miles.  The  next  return  to  the  perihelion  took  place 
in  September  or  October,  1851 ;  but  it  was  in  a  posi- 
tion very  unfavorable  for  observation,  and  conse- 
quently was  not  seen.  Moreover,  the  small  number 
of  observations  which  were  made  in  1846,  rendered 
it  extremely  difficult  to  determine  the  precise  period 
of  its  return,  otherwise  it  would  probably  have  been 
observed  in  1851.  The  next  and  last  return  took 
place  in  1857 ;  but  the  comet  was  accidentally  dis- 
covered by  Bruhns,  at  Berlin,  on  the  18th  of  March, 
and  its  identity  established  only  after  parabolic  ele- 
ments had  been  computed  which  were  very  similar 


250  TREATISE   ON   COMETS. 

to  those  of  Brorsen's  comet.  The  supposed  identity 
which  resulted  from  these  approximate  elements, 
was  subsequently  fully  confirmed  ;  and  the  comet  is 
now  ranked  among  those  of  short  period  whose 
future  movements  can  be  foretold  with  the  utmost 
precision.  It  passed  its  perihelion  on  the  29th  of 
March,  1857  ;  and  may  be  expected  to  return  again 
in  October,  1862. 

A  sixth  comet  of  short  period  was  discovered  by 
D'Arrest,  at  Leipsic,  on  the  27th  of  June,  1851.  It 
was  a  faint,  telescopic  object,  without  a  tail,  but 
having  a  distinct  and  well-defined  outline,  with  a 
point  of  greater  condensation  near  its  centre.  It 
passed  its  perihelion  on  the  8th  of  July,  1851 ;  and 
was  found  to  move  in  an  elliptic  orbit  corresponding 
to  a  period  of  revolution  of  2353  days,  or  nearly  six 
and  a  half  years.  The  next. return  was  therefore 
predicted  to  take  place  about  the  end  of  1857,  or 
beginning  of  1858  ;  and  the  comet  was  observed  at 
the  Cape  of  Good  Hope  in  December,  1857,  and  in 
January,  1858.  The  next  return  will  occur  in  June, 
1864. 

The  seventh  comet  of  short  period,  whose  orbit  is 
definitely  knowrn,  was  discovered  in  the  constella- 
tion Andromeda,  by  Mr.  Horace  P.  Tuttle,  at  the 
observatory  of  Harvard  College,  Cambridge,  Mass., 
on  the  evening  of  the  4th  of  January,  1858 ;  and  was 
subsequently  independently  discovered  by  Bruhns, 


TUTTLE'S   COMET.  251 

at  Berlin,  on  the  llth  of  the  same  month.  At 
the  time  of  its  discovery  it  appeared  like  a  faint  and 
diffused  nebula,  without  any  indications  either  of  a 
tail,  or  of  an  elongation  in  a  direction  opposite  to 
the  sun.  At  the  beginning  of  February  it  had  in- 
creased very  considerably  in  brilliancy,  and  pre- 
sented the  appearance  of  a  well-defined  elliptical 
nebula,  about  three  minutes  of  an  arc  in  diameter, 
with  a  distinct  increase  of  light  from  the  borders 
toward  the  centre.  It  continued  visible  until  near 
the  end  of  March  —  the  last  observation  reported 
from  any  part  of  the  world  being  that  made  at 
Cambridge,  Mass.,  on  the  23d  of  this  month.  When 
last  observed,  although  then  very  far  south,  and 
apparently  enveloped  in  the  vapors  near  the  horizon, 
it  nevertheless  appeared  distinct,  and  nearly  as  bril- 
liant as  when  it  was  first  discovered;  so  that,  in 
case  it  had  been  detected  at  some  observatory  in  the 
southern  hemisphere,  it  might  have  been  followed 
at  least  one  month  longer.  The  observations,  how- 
ever, which  were  made  in  Europe  and  in  the  United 
States  during  the  period  of  its  visibility  in  the  north- 
ern hemisphere,  have  enabled  astronomers  to  deter- 
mine the  elements  of  its  orbit  with  considerable 
precision. 

The  first  approximate  elements  of  the  orbit  of  this 
comet  were  found  to  bear  a  striking  resemblance  to 
those  of  the  second  comet  of  1790,  which  was  dis- 


252  TREATISE   ON   COMETS. 

covered  by  Mechain,  at  Paris,  on  the  9th  of  Janu- 
ary of  that  year.  Indeed,  so  close  was  this  resem- 
blance, that  the  identity  of  these  two  comets  was 
placed  beyond  a  doubt;  but  it  still  remained  to 
determine  whether  the  comet  had  not  previously 
returned  to  its  perihelion  unperceived,  during  the 
interval  of  sixty-eight  years  which  had  elapsed  be- 
tween 1790  and  1858.  It  was  found  by  direct  cal- 
culation of  elliptic  elements,  that  the  period  of 
revolution  is  about  thirteen  and  a  half  years ;  and, 
consequently,  that  the  comet  had  returned  to  the 
perihelion  no  less  than  five  times  between  1790  and 
1858.  It  might  be  expected,  however,  that  since 
the  orbit  of  the  comet  is  so  situated  that  the  per- 
turbations can  never  be  very  considerable,  the  comet 
would  have  been  observed  at  one  or  more  of  its 
intermediate  returns.  In  the  latter  part  of  1830, 
and  in  the  beginning  of  1831,  the  comet  was  favor- 
ably situated  for  observation,  both  in  respect  to  its 
apparent  angular  distance  from  the  sun,  and  its  bril- 
liancy. It  might  at  this  time  have  been  easily  ob- 
served in  the  east  before  sunrise ;  and  it  must  be 
supposed,  therefore,  that  no  search  was  made  for 
comets  in  that  portion  of  the  heavens  in  December, 
1830,  and  in  January,  1831.  At  the  other  three 
returns  of  the  comet  to  its  perihelion,  it  was  in  such 
close  proximity  to  the  sun,  and  —  on  account  of  its 
greater  distance  from  the  earth  —  so  diminished  in 


TUTTLE'S   COMET.  253 

brilliancy,  as  not  to  come  within  the  range  of  the 
optical  power  of  such  instruments  as  were  usually 
employed  in  searching  for  cometary  hodies.  It 
might  likewise  be  supposed  that  the  comet  would 
have  been  observed  at  some  return  to  its  perihelion 
previous  to  1790 ;  but  it  should  be  remembered  that, 
until  near  the  commencement  of  the  present  cen- 
tury, no  systematic  search  for  faint,  telescopic  comets, 
had  been  instituted;  and  that  its  discovery  at  a  date 
prior  to  1790,  could  only  have  been  the  result  of  a 
very  fortunate  accident. 

The  opposite  diagram  represents  the  appearance 
of  this  comet  on  the  31st  of  January,  1858,  and  may 
be  regarded  as  a  general  delineation  of  the  physical 
appearance  of  all  those  comets  which  have  no  tails, 
or  distinct  traces  of  any  considerable  elongation  in 
the  direction  of  the  sun. 

The  orbit  of  Turtle's  comet  is  an  ellipse,  corre- 
sponding to  a  periodic  time  of  5005  days,  or  a  little 
more  than  thirteen  years  and  eight  months.  When 
in  the  perihelion,  its  distance  from  the  sun  is 
100,000,000  miles.  Its  aphelion  distance  is  987,- 
000,000  miles.  It  passed  its  perihelion  on  the  28th 
of  February,  1858,  and  will  return  again  to  the 
same  point  about  the  end  of  November,  1871. 

Another  comet  of  short  period  is  that  of  Win- 
necke,  which  was  first  observed  by  Pons,  at  Mar- 
seilles, on  the  12th  of  June,  1819.  It  was  observed 
22 


254  TREATISE   ox   COMETS. 

during  a  period  of  only  forty  days,  and  from  the 
observations  thus  obtained,  Encke  found  that  its 
orbit  was  an  ellipse,  with  a  period  of  only  five  and 
a  half  years.  It  was  so  uncertain,  however,  that  no 
attempt  was  made  to  predict  its  future  returns  to  its 
perihelion ;  and,  consequently,  it  was  not  again  seen 
until  the  8th  of  March,  1858,  when  it  was  disco- 
vered by  Winnecke,  at  Bonn,  and  identified  by  a 
comparison  of  its  elements  with  those  of  the  comet 
of  1819.  It  was  observed  until  the  beginning  of 
June,  1858,  and  from  the  series  of  observations  it 
was  found  to  have  a  periodic  time  of  1830  days,  or 
almost  exactly  five  years.  It  has,  therefore,  next  to 
Encke's  comet,  the  shortest  period  which  has  yet 
been  discovered  as  belonging  to  such  bodies.  Its 
distance  from  the  sun,  when  in  its  perihelion,  is 
about  73,000,000  miles.  Its  aphelion  distance  is 
about  480,000,000  miles.  It  must,  therefore,  ap- 
proach very  near  the  orbit  of  Jupiter,  and  in  course 
of  time  will  be  subject  to  excessive  perturbations. 

This  comet  is  a  telescopic  object,  not  unlike 
Tuttle's  comet  in  its  general  appearance.  It  is  pro- 
bably much  larger  and  brighter,  but  in  form  bears 
a  close  resemblance  to  it.  It  passed  its  perihelion 
on  the  2d  of  May,  1858,  and  may  be  expected  to 
return  again  in  April,  1863. 

We  have  now  described  those  periodic  comets 
whose  orbits  are  supposed  to  be  known  with  great 


LEXELL'S   COMET.  255 

precision ;  but  there  are  others,  whose  periodic 
character  is  fully  as  decidedly  indicated,  but  which 
have  not  been  observed  to  a  sufficient  extent  for  a 
complete  investigation  of  their  elements.  A  revi- 
sion of  the  recorded  observations  of  former  comets 
by  modern  astronomers,  with  the  aid  of  the  refined 
operations  of  modern  mathematical  analysis,  has  led 
to  the  discovery  of  the  great  probability  of  several 
among  them  having  revolved  in  elliptic  orbits,  with 
periods  not  differing  to  any  considerable  extent 
from  those  of  the  periodic  comets  already  mentioned. 
The  fact  that  these  comets  have  not  been  re-observed 
at  their  successive  returns  to  their  perihelia,  may  be 
explained,  either  by  the  difficulty  of  observing  them, 
owing  to  their  unfavorable  positions,  and  the  cir- 
cumstance of  observers  not  expecting  their  reap- 
pearance, their  periodic  character  not  being  then 
suspected  ;  or  because  their  orbits  may  have  been  so 
completely  changed,  by  the  disturbing  action  of  the 
planets,  as  to  keep  them  continually  beyond  the 
limits  of  visibility  from  the  earth. 

A  singular  example  of  this  kind  is  found  in  the 
case  of  the  comet  which  appeared  in  1770,  and 
which  is  known  as  LexeWs  comet,  from  the  discoverer 
of  its  periodic  character.  It  is,  perhaps,  the  only 
instance  in  which  a  particular  comet  is  certainly 
known  to  have  appeared  in  the  system,  making  two 
revolutions  around  the  sun  in  an  elliptic  orbit  of 


256  TREATISE   ON    COMETS. 

short  period,  and  then  disappearing,  without  fur- 
nishing any  means  of  identifying  it  at  any  future  or 
previous  return.  This  comet  was  discovered  by 
Messier,  at  Paris,  on  the  14th  of  June,  1770,  and 
observed  till  the  2d  of  October  following.  It  passed 
its  perihelion  on  the  14th  of  August,  and  on  the  1st 
of  July  was  distant  from  the  earth  only  363  terres- 
trial radii,  or  about  six  times  the  distance  of  the 
moon. 

On  computing  the  elements  of  the  orbit  of  this 
comet,  it  was  found  to  be  impossible  to  represent 
the  observations  by  a  parabola.  Six  years  after  its 
disappearance,  Lexell  showed  that  the  observations 
could  be  satisfactorily  represented  by  an  ellipse, 
corresponding  to  a  period  of  about  five  and  a  half 
years.  This  was  certainly  a  remarkable  result, 
since  there  was  no  other  example  of  a  comet  of  less 
period  than  about  seventy-five  years ;  and,  more- 
over, the  fact  that  the  comet  had  not  been  seen  at 
previous  returns,  since  it  was  a  brilliant  object  in 
1770,  seemed  to  militate  strongly  against  Lexell's 
conclusions  in  regard  to  its  periodic  character.  The 
aphelion  distance  of  the  comet  was  found  to  be 
about  520,000,000  miles,  and  its  perihelion  distance, 
65,000,000  miles.  Its  orbit  is  inclined  to  the  orbit 
of  the  earth  by  an  angle  of  only  a  degree  and  a 
half;  and  it  follows,  therefore,  that  it  almost  lite- 
rally intersects  the  orbits  of  the  earth,  Mars,  and 


LEXELL'S   COMET.  257 

Jupiter.  This  fact  enabled  Lexell  to  remove  all 
doubts  in  regard  to  the  accuracy  of  his  results,  since 
it  could  be  easily  shown  that  at  its  previous  aphe- 
lion, in  1767,  the  comet  must  have  passed  Jupiter 
at  a  distance  fifty-eight  times  less  than  the  planet's 
distance  from  the  sun ;  and  that,  consequently,  it 
must  then  have  sustained  an  attraction  from  the 
great  mass  of  the  planet,  more  than  three  times 
more  energetic  than  that  of  the  sun,  at  the  same 
time.  It  follows,  also,  that  in  this  manner  its  former 
orbit  was  changed  into  an  ellipse  of  only  five  and  a 
half  years,  in  which  it  actually  moved  in  1770.  It 
is  highly  probable  that,  prior  to  1767,  its  orbit  was 
a  parabola,  and  that,  although  moving  in  an  elliptic 
orbit  from  1767  to  1770,  and  having  the  periodicity 
consequent  on  such  motion,  it  nevertheless  moved 
then  for  the  first  time  in  its  new  orbit,  and  had 
never  come  within  the  sphere  of  the  sun's  sensible 
attraction  anterior  to  this  epoch. 

The  return  of  the  comet  in  1776  could  not  be 
observed,  on  account  of  the  relative  situations  of 
the  earth  and  comet  at  that  time  —  the  latter  being 
wholly  enveloped  in  the  sun's  rays  or  in  the  strong 
twilight;  and  before  another  revolution  could  be 
accomplished,  its  orbit  was  again  completely 
changed.  Lexell  had  further  determined  that, 
since  the  comet  passed  through  its  aphelion,  which, 
as  already  stated,  nearly  intersected  the  orbit  of 
22*  R 


258  TREATISE   ON    COMETS. 

Jupiter,  at  intervals  of  a  little  more  than  five  and  a 
half  years,  it  having  been  in  that  position  in  1767, 
and  the  period  of  the  planet  being  less  than  twelve 
years,  the  planet,  after  a  single  revolution,  and  the 
comet  after  two  revolutions,  must  necessarily  again 
encounter  each  other  in  1779.  He  found  that  the 
comet  would  approach  Jupiter,  about  the  23d  of 
August,  1779,  within  a  distance  ninety-one  times 
less  than  the  distance  of  the  latter  from  the 
sun.  It  would,  therefore,  be  attracted  towards  the 
planet  by  a  force  which  was  at  least  200  times 
greater  than  the  sun's  attraction  at  the  same  point 
of  its  orbit ;  and,  consequently,  its  orbit  would,  in 
all  probability,  be  again  changed  into  a  parabola  or 
an  hyperbola.  It  would  thus  depart  for  ever,  for 
aught  we  know,  from  our  system,  to  visit  other 
suns  and  systems.  Lexell,  therefore,  announced 
that  in  1770  it  was  actually  describing  an  elliptic 
orbit  with  a  period  of  five  and  a  half  years;  but 
that,  owing  to  the  excessive  perturbations  produced 
by  Jupiter  in  1779,  it  might  be  expected  to  have 
already  made  its  final  disappearance. 

At  the  time  when  these  computations  were  made 
by  Lexell,  the  exact  methods  of  Lagrange  and  La- 
place, for  the  computation  of  perturbations,  had  not 
yet  been  developed.  Subsequently,  the  question 
here  propounded  was  resumed  by  the  latter  geom- 


LEXELL'S   COMET.  259 

eter,  and  a  general  solution  was  obtained  for  the 
following  definite  problem: 

The  actual  orbit  of  a  comet  being  given,  what  was  its 
orbit  before,  and  what  will  be  its  orbit  after  being  sub- 
mitted to  any  given  disturbing  action  of  a  planet  near 
which  it  passes  f 

Having  obtained  a  general  solution  of  this  prob- 
lem, he  applied  his  formula  to  the  particular  case 
of  Lexell's  comet,  and  showed  that,  before  sustain- 
ing the  disturbing  action  of  Jupiter  in  1767,  the 
comet  must  have  moved  in  an  ellipse  whose  greater 
axis  was  about  2,526,000,000  miles;  and,  conse- 
quently, that  its  period,  instead  of  being  five  and  a 
half  years,  must  have  been  forty-eight  and  a  half 
years.  He  showed  also  that  the  eccentricity  of  the 
orbit  of  the  comet  was  such  that  its  perihelion  dis- 
tance would  be  nearly  equal  to  the  mean  distance 
of  Jupiter,  so  that  it  could  never  have  been  visible. 
We  perceive,  therefore,  that  previous  to  1767,  the 
comet  experienced  excessive  perturbations  only  when 
near  its  perihelion.  It  was  further  demonstrated  by 
Laplace,  that  after  being  subjected  to  the  disturbing 
action  of  Jupiter  in  1779,  the  orbit  of  five  and  a  half 
years  was  changed  into  an  ellipse  whose  greater  axis 
was  about  1,400,000,000  miles,  thus  indicating  a  pe- 
riod of  twenty  years.  The  eccentricity  of  the  orbit 
of  the  comet  was  found  to  be  such  that  its  perihelion 
distance  was  more  than  double  that  of  the  planet 


260  TREATISE   ON    COMETS. 

Mars,  so  that  even  in  such  an  orbit  the  comet  could 
not  become  visible. 

The  question  rested  thus  for  nearly  half  a  century, 
when  the  investigations  of  Laplace  were  thoroughly 
revised  by  Le  Verrier.  He  found  that  the  observa- 
tions of  1770  were  not  sufficiently  accurate  and 
numerous  to  warrant  such  absolute  conclusions  ;  and 
showed  that  the  orbit  of  1770  is  subject  to  an  un- 
certainty comprised  between  certain  definite  limits ; 
and  also  that,  tracing  the  consequences  of  this  to  the 
positions  of  the  comet  in  1767  and  1779,  these  places 
are  subject  to  still  wider  limits  of  uncertainty. 
Thus  he  finds  that  the  observations  of  1770  can  be 
almost  equally  well  represented,  no  matter  whether 
the  comet  is  supposed,  in  1779,  to  have  passed  con- 
siderably outside,  or  considerably  inside,  of  Jupiter's 
orbit;  or  whether,  as  it  was  supposed  to  have  done, 
it  actually  passes  within  and  among  the  orbits  of  the 
satellites.  Finally,  he  deduces  the  following  gene- 
ral conclusions : 

1.  That  if  the  comet  had  passed  within  the  orbits 
of  the  satellites  of  Jupiter,  it  must  have  fallen  down 
upon  the  planet  and  coalesced  with  it — an  incident 
which  must  be  considered  highly  improbable,  though 
not  absolutely  impossible. 

2.  The  action  of  Jupiter  may  have  thrown  the 
comet  into  a  parabolic  or  hyperbolic  orbit,  in  which 
case  it  must  have  taken  its  final  departure  from  our 


L  K  x  E  L  L  '  s   COMET.  261 

system,  never  again  to  return  ;  except  it  may  be  by 
the  consequence  of  some  disturbance  of  its  motion 
produced  in  another  sphere  of  attraction. 

3.  It  may  have  been  thrown  into  an  elliptic  orbit, 
having  a  great  axis  and  long  period,  and  so  placed 
and  formed  that  the  comet  would   never  become 
visible — a  supposition  which  agrees  with  the  solution 
of  Laplace. 

4.  It  may  have  had  merely  its  elliptic  elements 
more  or  less  modified  by  the  action  of  the  planet, 
without  losing  its  character  of  short  periodicity  —  a 
result  which  is  perhaps  the  most  probable,  and  which 
would  render  it  possible  that  this  comet  may  still 
be  identified  with  some  one  of  the  many  comets  of 
short  period,  which  the  activity  and  sagacity  of  ob- 
servers are  continually  bringing  to  notice. 

In  order  to  facilitate  all  such  comparisons,  Le 
Verrier  has  given  a  table,  which  includes  all  the  pos- 
sible systems  of  elliptic  elements  of  short  period 
which  the  comet  could  have  assumed,  subject  to  the 
disturbing  action  of  Jupiter  in  1779,  and  taking  the 
observations  of  1770  within  the  limits  of  their  pro- 
bable errors,  but  which  it  will  be  unnecessary  to  add 
in  this  connection.  He  then  proceeds  to  demonstrate 
that  the  orbit  in  which  the  comet  moved  prior  to  the 
disturbing  action  of  Jupiter  upon  it  in  1767,  not 
only  could  not  have  been  an  ellipse  or  hyperbola, 
but  must  have  been  an  ellipse  whose  periodic 


262  TREATISE   ON   COMETS. 

time  was  considerably  less  than  had  been  deduced 
by  Laplace,  from  the  insufficient  observations  of 
Messier.  His  calculations  show,  that  before  that 
epoch,  the  perihelion  distance  of  the  comet  could 
not,  under  any  possible  supposition,  have  exceeded 
three  times  the  earth's  mean  distance  from  the  sun, 
and  was,  most  probably,  included  between  one  and 
a  half  and  two  times  that  distance  ;  and  that  the  ap- 
helion distance  could  not  have  exceeded  six  times 
the  same  distance,  a  magnitude  three  times  less  than 
that  assigned  to  it  by  the  calculations  of  Laplace. 

Having  thus  thoroughly  investigated  the  path  of 
Lexell's  comet,  both  before  and  after  its  successive 
changes,  from  the  action  of  Jupiter,  Le  Yerrier  went 
still  further,  and  endeavored  to  find  whether  any  of 
the  periodic  comets  now  known  can  be  supposed  to 
be  identical  with  this  comet.  In  accomplishing  this 
it  became  necessary  to  trace  the  paths  of  each  comet 
back  through  all  its  previous  revolutions  since  1779, 
making  due  allowance  for  the  disturbances  to  which 
it  was  subjected  by  the  action  of  the  planets  in  the 
interval  which  had  elapsed,  and  then  to  compare 
the  elements  thus  obtained  for  1779,  with  the  table 
of  possible  orbits  of  Lexell's  comet  which  had 
already  been  determined.  Should  one  of  these 
orbits  be  found  to  be  identical  with  that  of  the  comet 
in  question,  its  identity  with  Lexell's  comet  would 
be  inferred  with  the  highest  degree  of  probability; 


COMETS   OF   SHORT   PERIOD.         263 

but  if,  on  the  other  hand,  such  discrepancies  were 
found  to  prevail  as  must  exceed  all  possible  errors 
of  observation  or  calculation,  the  identity  could  not 
be  assumed.  It  was  found  in  this  way  that  none 
of  the  periodic  comets  hitherto  observed,  and  more 
especially  those  of  short  period,  can  be  identical 
with  Lexell's  comet,  however  strongly  some  of  the 
elements  of  their  present  orbits  might,  at  first,  seem 
to  indicate  such  a  presumption. 

Another  comet  of  short  period  was  discovered  by 
Blainplan,  at  Marseilles,  on  the  28th  of  November, 

1819,  and  was  observed  until  the  25th  of  January, 

1820.  From   the   observations   which  were    made 
during  this  period  of  two  months,  Encke  found  that 
its  orbit  was  an  ellipse,  corresponding  to  a  period 
of  revolution  of  about  five  years.    Clausen  considers 
this  comet  to  be  identical  with  one  which  appeared 
in  January,  1743.     It  passed  its  perihelion  on  the 
20th  of  November,  1819,  but  has  not  been  observed 
at  any  succeeding  return.* 

A  comet  which  was  discovered  by  Pigott,  at  York, 
in  1783,  was  shown  by  Burckhardt  to  move  in  an 
elliptic  orbit,  with  a  periodic  time  of  five  and  a  half 
years.  It  passed  its  perihelion  on  the  20th  of  No- 
vember of  that  year,  but  has  not  been  seen  since 
the  21st  of  December  following. 

On  the  26th  of  June,  a  telescopic  comet  was  dis- 
covered by  Peters,  at  Naples,  and  observed  till  the 


264  TREATISE   ON    COMETS. 

21st  of  July.  Its  orbit  has  been  calculated  by 
D'Arrest,  and  found  to  be  an  ellipse  with  a  period 
of  5804  days,  or  nearly  sixteen  years.  Later  com- 
putations by  Peters  have  assigned  to  it  a  periodic 
time  of  a  little  less  than  thirteen  years.  It  passed 
its  perihelion  on  the  1st  of  June,  1846,  and  was  pre- 
dicted to  return  again  in  the  first  part  of  1859.  It 
was,  however,  in  a  position  very  unfavorable  for  ob- 
servation in  the  northern  hemisphere,  and  no  obser- 
vation of  it  since  1846  has  yet  been  announced. 

These  are  the  only  additional  comets  whose  period 
is  less  than  sixteen  years,  which  have  hitherto  been 
detected ;  but  there  are  several  with  periods  of  seventy 
years  and  upward,  whose  orbits  are  supposed  to  be 
known  with  considerable  accuracy,  although  the 
comets  have  only  been  observed  at  one  return  to  the 
perihelion.  Among  these  might  be  noticed  a  comet 
discovered  by  Pons,  at  Marseilles,  on  the  20th  of 
July,  1812,  for  which  Encke  found  a  periodic  time 
of  about  seventy-five  and  a  half  years ;  also,  one 
discovered  by  Olbers,  at  Bremen,  on  the  6th  of  March, 
1815,  whose  orbit,  calculated  by  Bessel,  proved  to 
be  an  ellipse  with  a  period  of  seventy  four  years. 
The  next  perihelion  passage  of  this  comet  is  pre- 
dicted to  take  place  on  the  9th  of  February,  1887, 
its  time  of  revolution  between  1815  and  1887  being 
diminished  about  two  years  by  the  action  of  the 
planets.  A  comet,  discovered  byDe  Vico,  at  Rome, 


COMETS   OF  LONG   PERIOD.  265 

on  the  28th  of  February,  1846,  is  found  to  have  a 
period  of  between  seventy-two  and  seventy-three 
years  ;  and  one  discovered  at  Altona,  in  Denmark, 
by  Brorsen,  on  the  20th  of  July,  1847,  has  been  sup- 
posed to  move  in  an  elliptic  orbit  with  a  period  of 
seventy-five  years.  Still  another  comet,  belonging 
to  the  same  class,  was  discovered  by  "Westphal,  at 
Gottingen,  on  the  27th  of  June,  1852,  and  subse- 
quently by  Peters,  at  Constantinople.  Its  orbit  is 
found  to  be  an  ellipse,  corresponding  to  a  period  of 
about  seventy  years. 

On  the  25th  of  July,  1857,  a  faint  telescopic  comet 
was  discocered  by  Peters,  at  Albany,  N.  Y.,  which 
was  observed  until  the  20th  of  October,  following. 
Soon  after  its  discovery,  it  was  found  that  its  orbit  was 
an  ellipse,  with  a  period  of  about  250  years.  Its  aphe- 
lion distance  is  about  7,700,000,000  miles,  or  nearly 
5,000,000,000  miles  beyond  the  orbit  of  Neptune. 

These  are  the  only  comets  of  long  period  whose 
orbits  are  known  to  be  contained  within  the  present 
limits  of  the  planetary  system.  There  are,  however, 
many  on  record,  whose  periodicity,  although  not  yet 
in  any  instance  established  by  observations  made  at 
successive  returns  to  their  perihelia,  is,  nevertheless, 
so  unequivocally  indicated,  that  it  is  possible  to  ob- 
tain a  very  approximate  value  of  the  dimensions  of 
their  orbits,  from  the  observations  made  during  the 
time  of  their  visibility  at  a  single  perihelion  passage. 
23 


266  TREATISE   ox    COMETS. 

We  have  already  given  several  examples  of  this  class 
of  cometary  bodies  in  the  preceding  chapter. 

Of  all  the  comets  whose  orbits  have  been  com- 
puted, only  twenty-seven  have  been  definitely  ascer- 
tained to  move  in  elliptic  orbits,  and  thirteen  have 
been  found  to  move  in  hyperbolas. »  The  others 
must,  therefore,  be  supposed  to  have  moved  in  orbits 
which  were  either  parabolic,  or  in  which  the  eccen- 
tricity differed  so  little  from  that  of  a  parabola  as 
not  to  be  indicated  by  the  observations  which  have 
been  made.  Those  which  move  in  elliptic  orbits 
have  periods  of  revolution  varying  from  3500  to 
6000  years,  and  of  these  only  the  following  have 
been  identified  and  observed  at  more  than  one  return 
to  their  perihelia. 

XT  ,.  n  Period  of  Time  of  next  return 

revolution  in  days.  to  the  perihelion. 

Halley's     ....  27877  ....  February,  1912 

Encke's 1207  .     ,     .     .          "  1862 

Biela's 2421  ....          "  1866 

Faye's 2727  ....  March,  1866 

Brorsen's    .     •     .     .  2031  ....  October,  1862 

V Arrest's  ....  2340  ....  July,  1864 

Turtle's 5004  ....  November,  1871 

Winnecke's     .     .     .  1830  ....  April,  1863 

A  very  curious  circumstance,  \vhich  has  often  been 
noticed  in  connection  with  the  periodic  comets,  is 
that  those  already  known  may  be  arranged,  for  the 
most  part,  into  two  classes  —  one  including  those 
comets  whose  mean  distances  from  the  sun  are  all 


D i  v i s  i  o N   OF  -COMETS.  267 

nearly  the  same  with  those  of  the  small  planets  be- 
tween Mars  and  Jupiter;  and  the  other  class,  in- 
cluding those  whose  mean  distance  is,  in  every  case, 
yery  nearly  equal  to  that  of  Uranus.  It  has,  there- 
fore, been  conjectured  that  the  comets  composing 
the  first  class  may  have  formerly  constituted  one,  or 
at  most  two,  whose  orbit  or  orbits  were  contracted 
by  the  disturbing  action  of  Jupiter,  and  that  after- 
ward they  may  have  been  sub-divided — forming  the 
different  comets  which  belong  to  this  class  —  by  a 
process  yet  unknown,  but  probably  similar  to  that 
by  which  Biela's  comet  has  been  separated  into  two 
distinct  parts  almost  under  the  eyes  of  the  observers. 
This  is  the  theory  advanced  by  Professor  Stephen 
Alexander,  who  imagines  that  the  catastrophe  or  suc- 
cession of  catastrophes  which  divided  these  comets, 
must  have  been  of  a  very  ancient  date.  The  comets 
of  Peters  (1846)  and  Tuttle,  whose  periods  are  a 
little  more  than  thirteen  years,  may  perhaps  be  an 
exception  to  any  general  law  or  order  of  distance 
which  may  thus  be  supposed  to  exist. 

The  existence  of  a  group  of  small  planets  between 
Mars  and  Jupiter  has  long  since  been  accounted  for 
by  the  supposition  that  they  at  one  time,  although, 
perhaps,  almost  infinitely  remote,  constituted  a  single 
planet  which  revolved  around  the  sun  at  about  the 
average  of  their  mean  distances.  This  original 
planet  is  supposed  to  have  been  broken  into  almost 


268  T  K  E  A  T  I  S  E     ON     C  0  M  E  T  S  . 

innumerable  fragments  by  some  internal  convulsion 
of  nature,  thus  forming  a  group  of  small  planets  or 
fragments  revolving  at  different  distances,  but  within 
the  orbits  of  Mars  and  Jupiter.  In  this  way  thei\ 
the  common  origin  of  the  asteroid  planets  may  be 
considered  as  conclusively  established.  It  has,  ac- 
cordingly, been  conjectured  that  since  the  comets 
of  short  period  may  be  supposed  to  have  a  common 
origin  also,  the  latter  may  have  formed  from  por- 
tions of  the  original  planet;  and  that  thus  both  the 
asteroids  and  comets  of  short  period  may  have 
formerly  constituted  but  one  mass,  which  was  subr 
sequently  divided  by  some  unusual  and  violent  phy- 
sical convulsion.  When  the  cohesion  of  the  particles 
of  matter  which  constituted  the  original  mass,  was 
overcome  by  the  action  of  the  explosive  force,  the 
fragments  which  would  thus  be  thrown  off,  would 
be  projected  with  a  velocity  depending  on  their 
magnitude  and  density.  The  largest  fragments 
would  continue  to  move  on  in  orbits  differing  but 
little  from  that  of  the  original  planet,  while  the 
lighter  fragments  would  assume  orbits  more  or  less 
eccentric,  depending  on  the  velocity  of  projection. 
If  now  we  suppose  that  the  planet  was,  just  before 
its  dissolution,  composed  in  part  of  matter  in  a 
gaseous  form,  we  may  conclude  with  some  degree 
of  probability,  perhaps,  that  the  comets  of  short 
period  may  have  been  formed  in  this  manner.  The 


COMETS  AND  ASTEROID  PLANETS.     269 

very  difference  of  inertia  of  the  different  parts  of 
the  planet,  might  seem  to  have  caused  the  separa- 
tion, under  these  circumstances,  of  the  nebulous  or 
cornetary  matter,  from  that  of  greater  density ;  and 
while  portions  of  this  same  nebulous  material  were 
still  retained  by  the  larger  portions  of  the  original 
mass,  the  smaller  portions  would  retain  scarcely  any 
traces  of  it.  In  support  of  this  hypothesis,  the  fact 
has  been  cited  that  some  of  the  asteroid  planets  are 
actually  observed  to  be  surrounded  by  a  very  dense 
atmosphere,  or  nebulous  envelope. 

However  plausible  these  hypotheses  in  reference 
to  the  similarity  and  common  origin  of  the  asteroid 
planets  and  the  comets  of  short  period  may  seem  to 
be,  yet  we  cannot  by  any  means  regard  them  as 
having  anything  more  than  the  very  slightest  degree 
of  probability.  There  is  a  very  beautiful  theory  of 
the  formation  of  the  solar  system, — which  we  shall 
explain  hereafter, — by  which  the  origin  of  cometary 
worlds  is  illustrated  with  great  distinctness,  and 
which  presents  far  greater  claims  to  our  attentive 
consideration  than  any  other  which  has  ever  been 
devised.  Reference  is  here  made  to  the  nebular 
hypothesis  of  Laplace,  who,  in  his  theory  of  planet- 
ary genesis  from  rings  of  vapor  revolving  round  the 
sun,  in  which  matter  aggregates  into  spheres  around 
a  nucleus,  asserts  that,  according  to  the  hypothesis 
of  zones  of  vapor,  and  of  a  nucleus  increasing  by 


270  TREATISE   ON    COMETS. 

the  condensation  of  the  atmosphere  which  surrounds 
them,  the  comets  are  to  be  regarded  as  strangers  to 
the  planetary  system. 

When  we  consider  the  subject  of  the  comets  in  its 
fullest  extent,  it  may  indeed  be  said  that  there  is  no 
other  branch  of  astronomy  more  replete  with  inter- 
est, as  furnishing  the  most  sublime  examples  of  the 
achievements  of  the  human  intellect  in  the  investi- 
gation of  the  intricate  problems  here  presented.  It 
is  certainly  wonderful  to  notice  the  extreme  preci- 
sion of  the  calculations  of  modern  astronomers  re- 
garding the  orbits  of  these  chaotic  worlds,  by  which 
they  are  enabled  to  conclude  definitely  what  has 
been  the  past,  and  what  will  be  the  future  history 
of  any  particular  body.  Each  newly  discovered 
comet  is  at  once  subjected  to  the  ordeal  of  a  most 
rigorous  inquiry;  and  its  elements,  roughly  calcu- 
lated within  a  few  days  after  its  first  appearance, 
are  gradually  corrected  as  observations  accumulate, 
until  finally  the  exact  form  and  position  of  its  orbit 
in  space  are  determined.  Its  connection  or  identity 
with  any  comet  which  may  have  previously  appeared 
is  carefully  examined,  and  when  a  decided  similarity 
has  been  detected,  the  disturbances  due  to  the  action 
of  the  planets  during  the  interval  which  has  elapsed, 
are  investigated;  and  the  past,  thus  brought  into 
unbroken  connection  with  the  present,  is  made  to 
afford  substantial  ground  for  prediction  of  the  future. 


PROBABLE  COLLISION  WITH  PLANETS.  271 

The  increasing  attention  which  is  being  bestowed 
upon  this  branch  of  knowledge,  may  ultimately  in- 
crease the  number  of  periodic  comets  to  an  extent 
almost  beyond  conjecture ;  and  when,  even  now,  we 
come  to  consider  the  manner  in  which  the  orbits 
very  nearly  intersect  those  of  the  various  planets 
belonging  to  our  system,  it  may  indeed  seem  proba- 
ble that  the  time  will  come  when  some  one  of  these 
bodies  will  come  into  collision  with  some  member 
of  the  planetary  system.  How  far  the  welfare  of 
the  inhabitants  of  the  earth  may  be  dependent  upon 
any  such  event,  when  the  collision  is  not  directly 
with  our  planet,  it  will  be  unnecessary  to  attempt  to 
determine ;  and  let  it  suffice  to  know  that  calcula- 
tions, based  on  the  theory  of  probabilities,  and  of 
the  most  positive  character,  show  that  there  is  but 
one  chance  in  281,000,000,  that  the  earth  will  come 
in  collision  with  one  of  these  bodies.  The  probable 
effect  of  such  a  catastrophe,  admitting  its  possi- 
bility, has  already  been  discussed  in  a  previous  con- 
nection. For  some  of  the  planets,  the  probability 
of  coming  in  contact  is  much  less  than  in  the  case 
of  the  earth ;  and  for  others  it  is  even  greater.  It 
has  indeed  been  asserted  that  Mars  was  struck  by  a 
comet  in  1315  or  1316,  or  that  there  was,  at  least, 
an  appulse,  if  not  an  actual  collision.  What  may 
have  been  the  effect  produced  on  the  comet,  in  case 
this  event  did  actually  take  place,  we  have  no  means 


272  TREATISE   ON   COMETS. 

of  determining ;  but  in  respect  to  Mars,  there  is  no 
indication  in  its  motions,  or  in  those  of  the  neigh- 
boring planets,  that  it  has  ever  suffered  disturbance 
from  any  such  source. 

It  might  also  be  expected,  considering  the  great 
number  of  comets  which  are  supposed  to  be  included 
within  the  solar  system,  that  a  collision  would  occa- 
sionally take  place  between  two  of  these  bodies. 
The  orbits  of  Encke's  and  Biela's  comets  actually 
intersect  each  other,  and  it  is  highly  probable  that 
at  some  future  time  they  may  meet  in  this  point. 
Should  this  event  take  place  in  the  month  of  Octo- 
ber, it  is  possible  that  the  inhabitants  of  the  earth 
may  witness  the  extraordinary  spectacle  of  an  en- 
counter between  these  two  bodies ;  and,  it  may  be, 
of  their  reciprocal  penetration  and  amalgamation, 
or  of  their  destruction  by  means  of  exhausting  ema- 
nations. In  view  of  this  fact,  it  has  most  justly 
been  stated  that  events  of  this  nature,  resulting 
either  from  deflection  occasioned  by  disturbing 
masses  orprimevally  intersecting  orbits,  must  have 
been  of  frequent  occurrence,  in  the  course  of  millions 
of  years,  in  the  immeasurable  regions  of  ethereal 
space.  Still,  they  must  be  regarded  as  isolated  oc- 
currences, exercising  no  more  general  or  alterative 
effects  on  the  order  and  harmony  of  the  celestial 
motions,  than  the  breaking  forth  or  extinction  of  a 
volcano,  within  the  limited  sphere  of  our  earth. 


PHYSICAL  CHARACTER  OF  COMETS.     273 


CHAPTER  IV. 

PHYSICAL  CONSTITUTION  OF  COMETS PERTURBATIONS  OF  THE  COMETS 

BY  THE  PLANETS THE  MASSES  OF    COMETS  —  EFFECT    OF    INTENSE 

HEAT    AND  COLD  ON  THEIR  FORM LIGHT    OF    COMETS TAILS    OF 

COMETS THEORY  OF  THE  FORMATION   OF    THE  TAIL INFLUENCE 

OF    THE  TAIL    ON    THE    MOTION    OF    THE    NUCLEUS    IN    ITS    ORBIT 

ORIGIN     OF     COMETS FORMATION      OF     NEW     WORLDS  —  NEBULAR 

HYPOTHESIS. 

WE  have  already  stated  briefly  the  general  charac- 
teristics by  which  the  comets  are  distinguished  from 
the  other  members  of  the  solar  system,  and  have 
also  explained  the  terms  by  which  each  part  of  a 
comet  is  designated.  It  is  proposed,  therefore,  in 
this  connection,  to  consider  more  fully  the  physical 
constitution  of  these  wandering  bodies,  the  effect 
of  the  various  degrees  of  temperature  to  which  such 
objects  may  be  subjected,  and,  finally,  their  probable 
origin.  The  theory  of  their  motions  we  have  found 
to  be  in  every  respect  complete,  but  in  the  case  of 
their  physical  constitution  it  is  far  different.  The 
data  here  presented  are  by  no  means  of  a  positive 
character,  and  the  conclusions  which  may  be  derived 
must  be  received  with  due  allowance.  There  may, 
indeed,  be  comets  in  existence  which  bear  some 
slight  resemblance  to  the  planets  in  their  physical 


271  TREATISE   ON    COMETS. 

constitution,  and  which,  were  it  not  for  the  great 
eccentricity  of  their  orbits,  might  justly  be  regarded 
as  planetary  worlds.  But  by  far  the  greater  number 
of  the  comets,  and  more  especially  those  which  are 
visible  only  through  a  telescope,  are  seen  generally 
as  clouds  or  masses  of  vapor,  with  or  without  a  train 
of  fainter  light. 

It  has  long  since  been  noticed  that  comets  might 
pass  directly  between  the  eye  of  the  observer  and 
stars  of  the  sixth  and  seventh  magnitude,  without 
diminishing  their  light ;  and  that,  in  every  such  case, 
the  effect  produced  on  the  light  of  the  star  would 
not  be  greater  than  might  be  expected  if  it  were 
shining  through  a  very  slight  fog.  There  are  nu- 
merous cases  of  this  kind  on  record,  all  of  which 
tend  to  show  that  the  comets  are  composed  of  matter 
in  a  state  of  the  very  greatest  attenuation.  From 
considerations  such  as  this,  and  the  fact  that  the 
light  of  the  stars  are  greatly  enfeebled  by  that  of 
the  full  moon  shining  through  our  atmosphere, 
attempts  have  been  made  to  determine  the  probable 
masses  of  some  of  the  comets ;  and  the  results  ob- 
tained have  invariably  indicated  that  they  contain 
but  a  very  small  quantity  of  matter.  In  some  cases 
it  has  been  found  that  the  absolute  quantity  of 
matter  in  the  entire  comet  was  almost  inconceivably 
small;  and  that,  consequently,  the  comet  was  so  ex- 
tremely attenuated  that  the  molecules  of  which  it  is 


PHYSICAL  CHAKACTER  OF  COMETS.     275 

composed  were  completely  isolated,  and  destitute  of 
mutual  elastic  reaction.  These  results,  however, 
must  be  received  with  considerable  allowance  for 
the  insufficiency  of  the  data  on  which  they  are 
based.  It  is  by  no  means  certain  that  a  central  pas- 
sage of  a  comet  over  a  star  will  not  almost  invari- 
ably cause  a  complete  occultation.  In  those  cases 
in  which  stars  have  been  seen  through  comets,  they 
have  usually  been  situated  barely  within  the  exterior 
nebulosity  or  envelope,  and,  consequently,  not  under 
circumstances  favorable  to  indicate  the  precise 
density  of  the  cometary  matter.  In  April,  1857,  the 
complete  occultation  of  a  star  by  Brorsen's  comet 
was  observed  at  Florence ;  and  a  few  months  later 
the  same  phenomenon,  produced  by  another  comet, 
was  observed  at  Altona,  in  Denmark.  In  both  cases 
the  comet  was  a  telescopic  object,  and  belonged  to 
the  class  of  diffused  nebulosities.  Thus,  although 
there  were  no  indications  that  the  comet  was  really 
solid  at  the  centre,  yet  the  density  of  the  nucleus 
was  such  as  to  prevent  a  passage  of  the  light  of  the 
star.  There  can  be  no  doubt  but  that  many  of  the 
comets  have  actually  a  solid  nucleus  surrounded  by 
an  immense  nebulous  atmosphere.  The  solid  part, 
however,  may  no,  t  exceed  a  few  miles,  or  perhaps  a 
feAV  hundred  miles,  in  diameter,  while  the  nebulous 
envelope,  or  cometic  atmosphere,  may  be  of  such 
enormous  dimensions  as  to  appear  as  if  constituting 


276  TREATISE   ON   COMETS. 

the  whole  matter  of  the  comet.  In  this  case  the 
comet  might  pass  almost  centrally  over  a  star  and 
yet  not  affect  its  brilliancy  very  materially,  or  cause 
a  complete  occupation.  It  is  evident,  therefore, 
that  the  combination  of  circumstances  which  would 
alone  furnish  a  sufficient  test  of  the  density  of  a 
comet,  by  passing  apparently  over  a  star,  is  of  rare 
occurrence.  It  would  be  necessary  that  the  centre 
of  the  head  of  the  comet,  although  very  small, 
should  pass  critically  over  a  star,  in  order  to  ascer- 
tain whether  the  latter  is  visible  through  it. 

In  September,  1832,  Biela's  comet  was  observed 
to  pass  directly  over  a  small  cluster  of  telescopic 
stars  of  the  sixteenth  or  seventeenth  magnitude ; 
and  the  entire  cluster  remained  distinctly  visible, 
thus  affording  a  striking  proof  of  the  extreme  trans- 
lucency  of  the  matter  of  which  this  comet  consists. 
The  most  trifling  fog  or  haze  in  the  earth's  atmo- 
sphere would  have  entirely  effaced  this  group  of 
stars,  yet  they  continued  visible  through  a  thickness 
of  cometic  matter  which,  at  its  central  part,  must 
have  been  nearly  50,000  miles.  In  this  case,  there- 
fore, the  comet  could  not  have  had  a  solid  nucleus 
of  any  considerable  size,  but  it  does  not  follow  that 
a  similar  phenomenon  could  have  been  observed  as 
a  general  result  for  all  comets.  It  is  certainly  known 
that  many  of  the  comets  which  have  hitherto  ap- 
peared have  a  solid  and  compact  nucleus;  and  we 


PHYSICAL  CHARACTER  OF  COMETS.     277 

may,  therefore,  conclude,  that  while  there  are  many 
comets  without  any  nucleus,  properly  so  called,  there 
are  some  with  a  nucleus  which,  perhaps,  may  be 
transparent ;  and  others  more  brilliant  than  planets, 
having  a  nucleus  which  is  solid  and  opaque.  In 
those  comets  in  which  the  appearance  of  a  nucleus 
is  most  distinctly  marked,  the  nebulous  envelope, 
instead  of  a  uniform  or  of  a  progressively  increasing 
brilliancy  from  the  circumference  toward  the  centre, 
is  often  apparently  composed  of  one  or  more  rings 
or  strata,  many  thousand  miles  in  thickness,  alter- 
nating with  fainter  portions — thus  presenting  an  ap- 
pearance not  unlike  what  would  be  presented  to  a 
spectator  viewing  from  a  distance  our  earth  with  its 
different  layers  of  clouds,  one  above  another,  sepa- 
rated by  portions  of  transparent  atmosphere. 

It  appears  from  such  considerations  that,  as  a 
general  result,  the  absolute  quantity  of  matter  con- 
tained in  any  of  the  comets  which  have  hitherto 
appeared,  must  be  extremely  small  when  compared 
with  that  of  the  planets  belonging  to  our  system, 
excepting,  perhaps,  some  of  the  group  of  asteroid 
planets  between  Mars  and  Jupiter.  That  they  are 
thus  constituted  is  still  further  evident  from  the  fact, 
that  for  ages  they  have  travelled  among  the  planets 
and  their  satellites  without  producing  any  disturb- 
ances which  have  yet  been  detected.  An  example 
of  this  kind  has  already  been  given  in  the  case  of 
24 


278  TREATISE   ON   COMETS. 

Lexell's  comet  of  1770,  which  passed  directly  among 
the  satellites  of  Jupiter,  without  in  the  least  dis- 
arranging that  system.  The  comet,  however,  was 
subjected  to  the  most  excessive  perturbations  by 
Jupiter,  its  orbit  being  completely  changed.  We 
have  also  numerous  instances  011  record,  in  which 
comets  have  approached  very  near  some  of  the 
smaller  planets  of  the  system  without  producing 
the  slightest  perceptible  disturbance,  but  in  which, 
without  exception,  the  comet  has  been  drawn  very 
considerably  from  its  previous  orbit.  According  to 
the  law  of  universal  gravitation  first  discovered  by 
Newton,  and  subsequently  demonstrated  to  the  very 
fullest  extent,  as  the  great  law  or  bond  which  unites 
the  various  bodies  composing  the  material  universe, 
by  the  researches  of  Lagrange,  Laplace,  and  others, 
every  portion  of  matter  attracts  and  is  attracted 
directly  as  the  quantity  of  matter,  and  inversely  as 
the  square  of  the  distance  from  the  attracting  body. 
This  is  the  law  of  universal  gravitation ;  and  it  may 
be  perceived  at  once,  from  the  mere  statement  of  the 
law,  that  the  comets  must  be  greatly  attracted  from 
their  regular  course  by  the  various  bodies  of  the 
systems  through  which  or  near  which  they  pass. 
The  disturbances  to  which  they  are  thus  subjected, 
and  to  which  we  have  briefly  alluded  in  the  pre- 
ceding chapter,  are  called  the  perturbations. 

To  attempt  an  explanation  of  the  manner  in  which 


PERTURBATIONS   OF     COMETS.       279 

the  perturbations  of  the  comets  maybe  determined, 
would  require  a  higher  degree  of  mathematical 
knowledge  than  can  be  expected  of  those  for  whom 
these  explanations  are  intended;  and  we  must  there- 
fore be  contented  with  a  mere  statement  of  the 
general  features  of  the  problem  thus  presented. 
If  the  system  were  composed  of  simply  the  sun  and 
one  planet,  the  latter  would  describe  an  exact  ellipse 
around  the  former,  or  rather  both  around  their  com- 
mon centre  of  gravity,  and  continue  to  perform  its 
revolutions  in  an  unchanged  orbit  through  all  time. 
But  if  now  we  add  to  the  system  a  third  body,  the 
attraction  of  this  third  body  will  draw  both  of  the 
former  bodies  out  of  their  mutual  orbits,  and  by 
acting  on  them  unequally,  will  disturb  their  relation 
to  each  other,  and  will  completely  efface  all  traces 
of  exact  elliptic  motion  either  around  one  another, 
or  about  a  fixed  point  in  space.  The  disturbance 
thus  produced  is  not  due  to  the  whole  attraction  of 
the  third  body,  but  rather  to  the  difference  of  its 
attractions  on  the  two  which  we  have  supposed  to 
have  originally  constituted  the  system. 

In  the  solar  system,  when  compared  with  the  sun, 
the  masses  or  weights  of  the  different  comets  and 
planets  composing  the  system,  are  extremely  minute. 
Their  attractions  on  each  other,  therefore,  are  all 
very  feeble  when  brought  into  comparison  with  the 
presiding  central  power  of  the  sun,  and  the  effects 


280  TREATISE   ON   COMETS. 

of  their  disturbing  forces  are  proportionally  small. 
This  fact  enables  us,  in  determining  the  perturba- 
tions of  any  particular  body,  to  estimate  each  of  the 
effects  of  the  disturbing  bodies  separately,  as  if  the 
others  did  not  take  place,  without  causing  any  error 
in  the  final  results  beyond  what  may  be  considered 
necessarily  incident  to  a  first  approximation.  It 
follows  directly  from  the  primary  relations  between 
forces  and  the  motions  which  they  produce,  that 
when  a  number  of  very  minute  forces  act  at  once  on 
a  system,  the  combined  effect  is  the  sum  or  aggre- 
gate of  their  separate  effects,  at  least  within  such 
limits,  that  the  original  relation  of  the  parts  of  the 
system  shall  not  have  been  materially  changed  by 
their  joint  action.  This,  in  the  case  of  the  mutual 
attractions  of  the  planets  and  comets,  may  be  sup- 
posed to  be  true  for  a  limited  portion  of  time,  but 
when  the  orbits  may  have  become  slightly  changed, 
it  becomes  necessary  to  take  account  of  these  changes 
in  determining  the  subsequent  effects.  It  is  evident, 
however,  that  in  investigating  the  disturbing  influ- 
ence of  the  several  planets  on  any  particular  comet, 
we  need  only  consider  the  case  of  a  single  disturb- 
ing planet  at  a  time,  and  having  found  their  indi- 
vidual effects,  the  sum  of  these  will  be  the  total 
effect.  The  problem  thus  reduces  to  that  of  three 
bodies;  a  predominant  central  body,  corresponding 
to  the  sun,  a  disturbing  body,  corresponding  to  a 


PERTUBATIONS   OF   COMETS.         281 

planet,  and  a  disturbed  body,  or  the  comet.  It  is 
here  supposed  that  the  mass  of  the  comet  is  small 
when  compared  with  either  that  of  the  sun  or  planet, 
which  has  hitherto  invariably  been  the  case ;  and  it 
may,  perhaps,  be  stated  as  a  general  fact,  that  no 
comet  will  probably  ever  appear  which  will  render  it 
necessary  to  take  into  account  its  action  on  any  of 
the  planets. 

Since  the  orbits  of  the  comets  are  inclined  to  the 
orbit  of  the  earth  at  all  possible  angles,  while  the 
orbits  of  the  other  planets  are  all  inclined  to  this 
within  very  narrow  limits,  excepting,  of  course,  some 
of  the  asteroid  planets,  it  follows  that  for  different 
comets  the  effect  produced  by  the  several  planets  on 
the  different  elements  will  be  greatly  dissimilar. 
For  example,  if  a  comet  happened  to  pass  near  a 
planet  when  in  the  vicinity  of  the  node  of  the  former, 
the  attraction  of  the  latter  might  produce  a  great 
change  in  the  inclination  of  its  orbit.  If  the  comet, 
on  the  contrary,  were  far  distant  from  either  node 
of  its  orbit,  the  effect  would  be  to  change  the  place 
of  the  node  without  affecting  very  considerably  the 
inclination.  In  the  case  of  a  comet  whose  orbit  is 
greatly  inclined  to  the  orbit  of  the  earth,  since  its 
distance  from  the  planets  in  the  remoter  parts  of  its 
orbit  will  be  much  greater  than  if  it  were  less  in- 
clined, the  perturbations  will  be  proportionally 
smaller.  It  is  evident,  therefore,  that  those  comets 
24* 


282  TREATISE   ON   COMETS. 

whose  orbits  have  the  greatest  inclination  to  those 
of  the  planets,  are  subject  to  the  least  disturbance ; 
while  those  which  move  very  nearly  in  the  plane  of 
these  orbits,  or  else  in  that  which  would  result  as  a 
mean  between  the  orbits  of  the  principal  planets, 
would,  on  account  of  the  great  length  of  their  orbits — 
which  would  bring  them  into  the  vicinity  of  several 
different  planets  —  be  subject  to  much  greater  dis- 
turbance. 

In  computing  the  perturbations  of  a  comet  by  a 
planet,  the  attractive  force  of  the  planet  on  the 
comet,  depending,  of  course,  on  their  mutual  dis- 
tance, is  resolved  into  three  parts  called  components, 
each  acting  in  the  direction  of  one  of  the  co-ordi- 
nate axes,  already  explained  in  the  preceding  chap- 
ter.* The  amount  of  the  disturbance  of  the  motion 
of  the  comet  in  the  direction  of  the  three  rectangular 
axes  is  thus  obtained ;  and  it  is  only  necessary  to 
add  them  to  the  rectangular  co-ordinates  of  the 
comet  computed  under  the  supposition  of  an  undis- 
turbed orbit,  or  subtract  them  from  these,  as  the 
case  may  be,  in  order  to  find  the  actual  co-ordinates 
of  the  comet  at  any  given  instant.  In  this  way  the 
perturbations  are  to  be  computed  from  day  to  day, 
or,  when  they  are  not  very  large,  at  intervals  of 
twenty  or  forty  days,  or  even  longer,  correcting  the 
elements  of  the  orbit,  from  time  to  time,  by  means 

*  See  page  170. 


PERTURBATIONS   OF   COMETS. 

of  the  variations  of  the  co-ordinates  already  ob- 
tained. Assuming  then  the  elements  of  the  orbit 
to  be  accurately  determined  from  observations  for 
any  given  epoch,  the  perturbations  or  variations  of 
the  co-ordinates  must  be  computed  at  regular  inter- 
vals, as  above  stated,  for  the  whole  period  during 
which  it  is  required  to  trace  the  motions  of  the 
comet.  The  same  process  must  be  performed  for 
each  disturbing  planet,  and  the  sum  of  the  separate 
variations  for  each  planet  will  give  the  total  amount 
of  the  perturbations.  Having  obtained  these  values, 
and  having  corrected  the  assumed  elements  accord- 
ingly, we  obtain  finally  the  elements  of  the  orbit  in 
which  the  comet  is  moving  at  the  instant  for  which 
its  true  orbit  is  required.  The  values  of  the  dis- 
turbances thus  formed  are  called  the  special  pertur- 
bations of  the  comet.  In  the  case  of  the  planets  it 
is  possible  to  compute  the  variations  of  their  elements 
due  to  their  mutual  attractions  for  an  indefinite  num- 
ber of  revolutions.  The  formulae,  however,  which  give 
these  variations,  are  so  affected  by  the  eccentricity 
of  the  orbit  of  the  planet,  that  the  solution  is  possi- 
ble only  when  the  eccentricity  is  small.  The  varia- 
tions of  the  elements  thus  obtained  are  called  the 
general  perturbations,  and  can  be  computed  only  in 
the  case  of  the  planets  —  the  eccentricity  of  their 
orbits  being  small.  In  the  case  of  the  comets  the 
eccentricity  of  the  orbit  is  always  so  great  that  it  is 


284  TREATISE   ON   COMETS. 

impossible  to  compute  their  general  perturbations, 
or  for  an  indefinite  number  of  revolutions ;  and, 
consequently,  astronomers  are  enabled  to  determine 
only  the  variations  of  the  elements  of  their  orbits — 
assuming  the  elements  for  a  given  epoch  to  be 
known — for  a  series  of  dates  in  regular  progression. 
When  a  comet  passes  to  such  an  enormous  distance 
that  the  attraction  of  even  the  remote  planets  of 
the  system  becomes  insensible,  it  is  no  longer  neces- 
sary to  compute  the  variations  of  its  elements  as 
before,  since  it  may  then  be  supposed  to  revolve  in 
a  pure  ellipse  around  the  common  centre  of  gravity 
of  the  system,  which,  however,  coincides  very  nearly 
with  the  centre  of  the  sun.  It  is  supposed  to  revolve 
thus  in  an  orbit  which  may  be  determined  from  the 
assumed  elements  in  connection  with  their  variations 
or  perturbations  as  already  determined,  until  finally 
it  again  comes  .within  the  sphere  of  the  sensible 
attraction  of  the  planets,  when  the  disturbance  of 
its  motion  is  again  computed  for  a  regular  series  of 
dates,  precisely  as  before.  In  this  way,  therefore, 
the  computation  of  the  perturbations  of  a  comet 
may  be  performed  for  a  long  series  of  years,  or  even 
for  a  number  of  revolutions ;  but  the  process,  as  may 
be  readily  perceived,  is  very  laborious.  In  the  case 
of  the  periodic  comets  the  perturbations  are  com- 
puted only  to  the  date  of  their  next  appearance, 
since  it  would  be  useless  to  continue  them  further 


THE   MASSES   OF   COMETS.  285 

\rithout  correcting  the  elements  by  means  of  a  series 
of  observations  which  might  then  be  taken.  In  the 
case  of  a  comet  moving  in  an  ellipse  of  very  great 
eccentricity,  or  in  which  the  period  is  more  than 
1000  years,  and  also  in  the  case  of  a  comet  moving 
in  a  parabola  or  hyperbola,  it  is  necessary  only  to 
compute  the  perturbations  during  the  short  period 
in  which  it  is  visible  from  the  earth,  since  in  these 
cases  all  that  is  required  is  to  find  the  exact  elements 
of  the  orbit  in  which  the  comet  was.  moving  while 
visible. 

Such  is  a  general  statement  of  the  method  by 
which  astronomers  are  enabled  to  follow  the  comets 
in  their  long  journeys  through  space,  while  invisible 
to  the  inhabitants  of  our  earth,  and  to  predict,  in 
the  case  of  the  periodic  comets,  the  exact  period  of 
their  return.  To  be  able  to  solve  this  problem 
requires  the  very  highest  degree  of  mathematical 
knowledge ;  and  let  it  suffice  here  to  say  that  it  is 
one  of  the  most  difficult  problems  of  astronomy, 
while  the  amount  of  labor  required  in  making  the 
numerical  computations,  vastly  exceeds  what  may 
be  readily  conceived.  An  illustration  of  this  fact 
may  be  witnessed  in  the  case  of  Halley's  comet 
already  given. 

We  have  already  stated  ijiat  in  all  computations 
respecting  the  motions  of  the  comets,  their  masses 
have  been  neglected  as  being  inconsiderable  in  com- 


286  TREATISE   ON   COMETS. 

parison  with  those  of  the  sun  and  planets.  Their 
effect  in  the  planetary  system,  even  if  there  were 
not  other  facts  tending  to  the  same  result,  would 
alone  be  sufficient  to  indicate  that  this  supposition  is 
entirely  admissible.  These  bodies  are  continually 
passing  through  our  system,  and  often  in  the  imme- 
diate vicinity  of  or  directly  among  the  asteroid 
planets,  and  yet  do  not  cause  the  slightest  disturb- 
ance which  can  be  determined  by  accurate  observa- 
tion. The  physical  appearance  of  the  comet  affords 
also  conclusive  proof  that  their  masses  are  at  least 
small.  Various  attempts  have  been  made  to  deter- 
mine the  masses  of  these  bodies;  and  although  the 
results  obtained  exhibit  great  discrepancies,  yet  they 
all  indicate  conclusively  that  in  the  case  of  a  very 
great  majority  of  the  comets  the  mass  is  extremely 
small,  or,  in  reality,  almost  inconceivably  small. 
It  may,  perhaps,  seem  really  absurd  to  assert  that 
the  tails  of  some  of  the  largest  comets  have  not 
contained  a  hundred  pounds  of  matter,  yet  it  has 
been  asserted  that  even  the  largest  and  longest  did 
not  contain  more  than  a  few  ounces.  "We  are,  of 
course,  unable  to  deny  such  an  assertion,  since  we 
have  no  means  of  proving  the  contrary.  It  must  be 
admitted,  however,  that  this  is  an  extremely  small 
estimate,  and  that,  although  the  comets  are  in  reality 
of  extreme  tenuity,  and  consequently  have  no  con- 
sideiable  mass,  yet  that  they  contain  vastly  more 


INTENSE  HEAT  AND   COLD.          287 

matter  than  what  would  result  from  any  such  esti- 
mate. The  only  correct  and  reliable  method  of 
arriving  at  the  exact  value  of  the  mass  of  a  comet, 
is  by  means  of  the  disturbance  which  it  may  produce 
in  the  motions  of  a  planet  near  which  it  approaches ; 
and  until  such  a  phenomenon  is  observed,  no  definite 
knowledge  of  this  element  can  be  obtained. 

When  we  come  to  consider  the  ever- varying  tem- 
perature to  which  the  comets  are  subjected  in  their 
motions  through  space,  the  question  may  very 
naturally  arise  as  to  the  probable  effect  which  would 
thus  be  produced.  It  might  be  supposed  that,  as  a 
necessary  consequence,  when  a  comet  approaches 
the  sun,  the  continually  increasing  temperature 
which  may  be  supposed  to  affect  it,  would  cause  it 
to  expand  gradually  until  it  arrived  at  its  perihelion, 
when  its  absolute  dimensions  would  be  greatest. 
For  a  similar  reason  it  might  be  expected  that  in 
receding  from  the  sun,  the  temperature  being  sup- 
posed to  decrease  continually,  it  would  be  gradually 
contracted,  and  would  finally  appear  precisely  as 
before  its  perihelion  passage.  It  may,  on  this  ac- 
count, seem  strange  that  the  dimensions  of  a  comet 
are  observed  almost  invariably  to  be  enlarged  as  it 
recedes  from  the  source  of  heat,  and  more  especially 
in  the  case  of  those  telescopic  comets  which  have  no 
tail.  This  singular  and  somewhat  unaccountable 
phenomenon  has  been  explained  in  various  ways. 


288  TREATISE   ON   COMETS. 

Valz  ascribed  it  to  the  pressure  of  the  solar  atmo- 
sphere, supposed  to  extend  to  a  great  distance,  acting 
upon  the  comet.  He  supposes  that  the  atmosphere, 
being  more  dense  than  the  sun,  compressed  the 
comet  and  diminished  its  dimensions ;  while,  at  a 
greater  distance,  being  relieved  from  this  coercion, 
the  body  again  expanded  to  its  natural  bulk.  In 
order  to  test  the  validity  of  this  hypothesis,  probable 
values  were  assumed  for  the  density  of  the  solar 
atmosphere  and  the  elasticity  of  the  comet;  and  the 
variations  of  the  bulk  of  a  comet,  deduced  in  accord- 
ance with  this  assumption,  exhibited  a  remarkably 
close  agreement  with  the  observed  change  in  its 
dimensions.  It  is  necessary,  however,  to  suppose 
also  that  the  comet  is  composed  of  an  elastic  gas  or 
vapor ;  and,  further,  that  it  is  impervious  to  the  solar 
atmosphere  through  which  it  moves,  both  of  which 
assumptions  are  inadmissible,  at  least  in  the  gene- 
rality of  cases.  Another  theory  which  has  been 
advanced  in  order  to  explain  the  fact  that  the 
dimensions  of  a  comet  are  enlarged  as  it  recedes 
from  the  source  of  heat,  is,  that  as  the  particles  or 
molecules  of  the  nebulous  matter  of  the  cornet  are 
distant,  and  held  together  by  so  feeble  a  power,  they 
may  revolve  to  a  certain  limited  extent  independ- 
ently of  each  other,  each  having  its  own  perihelion  ; 
and  that  thus  they  would  be  brought  nearer  to  each 
other  as  the}7  approach  the  sun,  and  separate  again 


INTENSE  HEAT  AND   COLD.          289 

further  and  further  as  they  depart  from  him.  This 
theory  assumes  that  the  mutual  cohesion,  or  mutual 
gravitation  of  these  particles  is  a  quantity  evanescent 
in  comparison  with  their  separate  gravitation  toward 
the  sun. 

Sir  John  Herschel  supposes  that  the  nebulous 
portion  of  the  comet,  or  that  portion  which  reflects 
the  sun's  rays,  is  of  the  nature  of  a  fog,  or  of  a  col- 
lection of  discrete  particles  of  a  volatile  fluid  floating 
in  a  transparent  medium.  During  the  comet's  ap- 
proach to  the  sun,  these  molecules  would  absorb  its 
rays  and  become  heated,  and  consequently  a  portion 
of  them  would  be  constantly  passing  from  the  liquid 
to  the  extremely  gaseous  or  invisible  state,  just  as  a 
fog  or  common  vapor  disappears  before  the  rising 
sun.  As  this  change  must  commence  from  without, 
and  must  be  propagated  toward  the  centre,  the  ulti- 
mate effect  would  be  a  diminution  of  the  visible 
bulk  of  the  comet.  As  the  comet  receded  from  the 
sun,  it  would  lose  by  radiation  the  heat  thus  acquired ; 
which,  in  accordance  with  the  general  analogy  of 
radiant  heat,  might  be  expected  to  escape  chiefly 
from  the  unevaporated  or  nebulous  mass  within. 
The  dimensions  of  the  latter  would,  therefore,  begin 
and  continue  to  increase  by  the  precipitation  above 
it  of  fresh  nebula,  just  as  fogs,  on  cold  still  nights, 
are  seen  to  form  at  first  on  the  surface  of  the  earth, 
and  as  the  heat  near  the  surface  becomes  dissipated, 


290  TREATISE   ON   COMETS. 

gradually  extending  upward.  A  comet  might  thus 
appear  to  enlarge  rapidly  in  its  visible  dimensions, 
while  its  real  volume  was  slowly  contracting  by  the 
general  abstraction  of  its  heat.  Herschel  supposes 
that  this  process  might  go  on  in  the  absence  of  any 
solid  or  fluid  nucleus ;  and  in  those  cases  in  which 
such  a  nucleus  exists,  the  increase  of  temperature 
in  the  vicinity  of  the  sun,  by  causing  an  evaporation 
from  its  surface,  would  afford  a  constant  and  copious 
supply  of  vapor,  which,  rising  into  its  atmosphere, 
and  condensing  at  its  exterior  parts,  would  tend 
still  more  to  dilate  the  visible  limits  of  its  nebulosity. 
In  this  manner  we  may  account  for  the  appearances 
which  have  been  noticed  in  the  head  of  certain 
comets,  where  a  stratum  void  of  nebula  has  been 
observed,  interposed,  as  it  were,  between  the  denser 
portion  of  the  nucleus,  and  the  coma  or  envelope. 
Another  theory  which  has  been  advanced  by  Her- 
schel, in  explanation  of  the  phenomenon  under  con- 
sideration, attributes  it  to  the  ethereal  medium  sur- 
rounding the  sun.  It  is  supposed  to  be  by  no  means 
improbable  that  the  region  in  which  the  earth  re- 
volves has  a  temperature  of  its  own  greatly  superior 
to  what  may  be  presumed  to  be  the  absolute  zero, 
and  even  to  what  may  be  produced  by  artificial 
means.  This  temperature  Herschel  supposes  to  be 
due  not  simply  to  the  radiation  of  the  stars,  but 
rather  to  the  contact  of  an  ether,  possessing  itself 


LIGHT   OF   COMETS.  291 

a  determinate  temperature,  and  tending,  like  all 
known  fluids,  to  communicate  this  temperature  to 
bodies  immersed  in  it.  ]^ow,  if  the  temperature  of 
the  ether  increases  in  approaching  the  sun,  which 
seems  to  be  a  necessary  consequence  —  regarding  it 
as  endued  with  the  ordinary  relations  of  fluids  to 
heat  —  an  obvious  explanation  is  furnished  of  the 
phenomenon  under  consideration.  A  body  of  such 
extreme  tenuity  as  a  comet,  may  be  presumed  to 
assume  very  readily  the  temperature  of  the  ether 
by  which  it  is  enveloped;  and  the  vicissitude  of 
warmth  and  cold  thus  experienced  may  ultimately 
convert  it  into  transparent  vapor,  and  again  pre- 
cipitate the  nebulous  substance,  just  as  fogs  are 
dissipated  by  an  increase  of  atmospheric  tempera- 
ture, not  by  abstracting  or  annihilating  its  aqueous 
particles,  but  by  causing  them  to  assume  the  elastic 
and  transparent  state,  which,  when  the  temperature 
falls,  they  again  lose,  and  appear  in  the  formation 
of  another  fog. 

Another  important  consideration  immediately  con- 
nected with  this  branch  of  our  subject,  is  whether 
the  comets  shine  partially  by  their  own  proper  light, 
or,  in  other  words,  whether  they  are  to  be  regarded 
as  to  a  limited  extent  self-luminous.  If  a  comet 
should  have  a  solid  and  opaque  nucleus  of  any  con- 
siderable size,  it  would  follow  as  a  necessary  con- 
sequence, in  case  it  did  not  have  a  light  of  its  own, 


292  TKEATISE   ON    COMETS. 

that  it  would  exhibit  the  phases  of  the  moon  when 
near  the  sun.  The  fact,  however,  that  these  have 
rarely,  if  ever,  been  certainly  observed,  and  also 
that  the  comets  are  known  to  be  composed  of 
nebulous  matter  in  an  extremely  attenuated  con- 
dition, has  rendered  it  uncertain  whether  they  shine 
by  reflected  light,  like  the  planets,  or  by  virtue  of 
their  being  self-luminous,  like  the  sun  and  stars. 
A  mere  mass  of  nebulous  matter,  not  itself  lumi- 
nous, but  rendered  visible  by  reflected  light,  would 
not  exhibit  phases  similar  to  those  of  the  moon  and 
interior  planets.  Its  imperfect  opacity  would  permit 
the  solar  light  to  affect  its  constituent  parts  through- 
out its  entire  depth —  so  that,  like  a  thin,  fleecy 
cloud,  it  would  not  appear  as  if  superficially  illumi- 
nated, but  as  receiving  and  reflecting  light  through 
all  its  dimensions,  both  externally  and  internally. 
If  we  suppose  the  comets  to  be  self-luminous,  it 
might  be  expected  that  they  would  be  visible  in  the 
remote  parts  of  their  orbits  until  their  distance  was 
absolutely  so  great  as  to  render  their  apparent 
diameter  inappreciable.  It  is  an  established  pro- 
perty of  self-luminous  bodies,  that  when  viewed 
from  any  distance  whatever,  they  will  appear  of  the 
same  intrinsic  splendor,  or,  in  other  words,  equal 
portions  of  the  apparent  surface  at  different  distances 
will  be  equally  brilliant.  Thus  the  sun,  when  seen 
from  Neptune,  the  exterior  planet  of  our  system, 


LIGHT   OF   COMETS.  293 

must  appear  as  bright  as  when  seen  from  the  earth. 
The  absolute  quantity  of  light  is  much  smaller,  since 
the  great  increase  of  distance  from  the  sun  in  pass- 
ing from  the  earth  to  that  planet  will  cause  the 
apparent  angular  diameter  of  the  sun  to  be  propor- 
tionally diminished.  It  is  evident,  therefore,  that 
although  equal  portions  of  his  disc,  as  seen  from 
these  planets,  may  be  equally  bright,  yet  the  total 
amount  of  light  received  at  each  will  be  vastly 
different.  It  follows  from  this,  that,  since  the  visi- 
bility of  a  self-luminous  object  does  not  depend 
upon  the  angle  which  it  subtends  as  long  as  it  is  of 
sensible  magnitude,  if  a  comet  shines  by  its  own 
light,  it  should  retain  its  brilliancy  as  long  as  its 
diameter  is  of  a  sensible  magnitude ;  and  that  even 
after  it  has  receded  to  such  a  distance  as  to  render 
its  apparent  diameter  inappreciable,  it  ought  to  be 
visible  like  the  fixed  stars,  and  should  only  vanish 
in  consequence  of  extreme  remoteness.  The  phe- 
nomena, however,  which  are  actually  observed,  do 
not  in  the  least  accord  with  any  such  hypothesis. 
The  comets  are  observed  to  become  gradually  fainter 
and  fainter  as  their  distance  from  the  earth  and  sun 
increases,  and  finally  become  invisible,  even  through 
the  most  powerful  telescopes,  while  their  apparent 
angular  diameter  is  very  considerable.  This  would 
seem  to  indicate  conclusively  that  they  shine  by 
reflected  light.  It  might  be  remarked  also,  in  this 
25* 


294  TREATISE   ON   COMETS. 

connection,  that  the  brilliancy  of  a  comet  computed 
from  day  to  day,  during  the  period  of  its  visibility, 
under  the  supposition  that  it  shines  by  reflected 
light,  is  found  to  coincide  very  closely  with  the 
actual  appearance  of  the  comet.  The  most  brilliant 
comets  have  ceased  to  be  visible  long  before  they 
had  receded  to  a  distance  of  500,000,000  miles ;  and 
a  very  great  majority  of  these  bodies  have  their 
perihelia  within  the  orbit  of  Mars,  while  none  have 
been  visible  from  the  earth  whose  perihelia  were 
exterior  to  the  orbit  of  Jupiter. 

The  facts  thus  established  are  entirely  incom- 
patible with  the  hypothesis  that  the  comets  are  self- 
luminous  bodies,  unless  it  is  supposed  that,  from 
some  physical  cause,  they  gradually  lose  their  lumi- 
nosity. The  phenomenon,  already  noticed,  of  the 
gradual  expansion  of  a  comet  in  receding  from  the 
sun,  might  perhaps  serve  to  account  partially  for  a 
diminution  of  their  luminosity.  The  luminous 
matter  thus  existing  in  a  less  condensed  state  might 
be  supposed  to  shine  with  a  proportionally  enfeebled 
splendor,  until,  at  length,  by  the  dilatation  of  the 
body,  the  light  would  become  so  reduced  as  to  be 
incapable  of  affecting  the  retina  of  the  eye  to  an 
extent  sufficient  to  produce  a  sensation.  The  ex- 
pansion of  the  comet,  however,  would  have  precisely 
the  same  effect  in  the  case  of  reflected  light ;  and 
it  is  evident,  therefore,  that  nothing  definite  can  be 


LIGHT   OF   COMETS.  295 

arrived  at  iii  this  manner,  except  by  experiment. 
Arago  has  submitted  to  examination  the  rate  in 
which  comets  increase  their  dimensions  in  receding 
from  the  sun,  and  has  found  the  corresponding 
diminution  of  intrinsic  splendor  which  would  arise 
from  such  a  cause.  It  remains  then  to  determine 
whether  the  circumstances  are  such  as  to  render  the 
brightest  comets  visible  beyond  the  orbit  of  Jupiter. 
This  Arago  proposed  to  determine  by  actual  experi- 
ment. Let  a  telescope  be  selected  having  a  large 
aperture  and  low  magnifying  power,  by  the  aid  of 
which  the  comet  may  be  observed  in  every  part  of 
its  visible  course.  Let  the  comet  be  observed  at 
some  determinate  distance  from  the  sun,  and  by 
varying  the  magnifying  power  of  the  telescope,  it 
may  be  made  to  assume  apparently  different  degrees 
of  brilliancy.  The  magnifying  power  of  the  tele- 
scope may  be  so  regulated  as  to  exhibit  the  comet 
with  precisely  that  degree  of  brilliancy  with  which 
it  would  appear  at  any  given  increased  distance 
from  the  sun,  if  observed  with  the  lowest  magnify- 
ing power  of  the  instrument,  under  the  supposition 
of  its  being  a  self-luminous  body,  and  losing  bright- 
ness by  reason  of  the  enlargement  of  its  dimensions. 
In  this  manner  the  actual  brilliancy  of  the  comet, 
if  self-luminous,  at  any  given  distance  from  the 
sun,  might  be  predicted;  and,  in  case  the  subse- 
quent observations  were  found  to  agree  completely 


296  TREATISE   ON    COMETS. 

with  the  prediction,  it  might  be  presumed  that  the 
comet  was  visible  by  means  of  its  own  light.  But 
if,  as  the  observations  actually  indicate,  the  brilliancy 
of  the  comet  at  different  distances  from  the  earth  is 
greatly  less  than  in  accordance  with  the  prediction, 
while  it  becomes  invisible  at  distances  at  which  its 
apparent  diameter  is  considerable,  then  it  may  be 
considered  as  conclusively  established,  that  the  body 
is  not  self-lurninous,  but  that  it  derives  its  light  from 
the  sun,  and  that  its  final  disappearance  from  our 
view  arises  from  the  extreme  faintness  of  the  light 
reflected  by  its  attenuated  matter. 

It  was  the  opinion  of  several  astronomers  that  the 
great  comet  of  1811  shone  by  inherent  light,  and  it 
was  asserted  that  the  rapid  variations  which  were 
observed  to  take  place  in  the  brilliancy  of  the 
nucleus,  together  with  the  flashes  of  light  which 
characterized  the  appearance  of  the  tail,  cannot  be 
explained  by  means  of  any  other  hypothesis.  In 
opposition  to  this  opinion,  it  may  be  remarked  that 
comets  have  been  seen  as  dark  spots  crossing  the 
sun's  disc,  and  also  that  their  light  exhibits  traces 
of  polarization.  On  the  18th  of  November,  1826, 
the  transit  of  the  dark  body  of  a  comet  across  the 
sun's  disc  was  observed  at  Marseilles  and  Yiviers, 
in  France ;  and  a  similar  phenomenon  was  observed 
at  various  places  on  the  6th  of  June,  1818.  There 
are  also  numerous  cases  on  record  in  which  dark 


LIGHT   or  COMETS.  297 

bodies  have  been  observed  in  their  transit  over  the 
sun's  disc,  which  were  undoubtedly  comets  having 
a  solid  and  opaque  nucleus.  In  these  cases  the 
comet  could  not  have  been  self-luminous,  since  it 
would  then  have  been  invisible ;  or,  at  least,  would 
have  appeared  as  a  bright  instead  of  a  dark  spot. 
Again,  it  is  found  that  all  direct  light  constantly 
divides  itself  into  two  points  of  the  same  intensity 
when  it  traverses  a  crystal  possessing  the  power  of 
double  refraction ;  while  reflected  light,  in  certain 
positions  of  the  crystal  through  which  it  is  made  to 
pass,  gives  two  images  of  unequal  intensity,  in  case 
the  angle  of  reflection  is  not  ninety  degrees ;  or,  in 
other  words,  it  is  polarized  in  the  act  of  reflection. 
In  general,  when  a  ray  of  light  is  reflected  from  any 
surface,  it  may  be  reflected  a  second  time  from 
another  surface,  and  may  also  be  made  to  pass  freely 
through  transparent  bodies.  If,  however,  a  ray  of 
light  be  reflected  from  any  surface  at  a  given  angle, 
— to  be  determined  by  experiment, — it  will  be  found 
to  have  been  rendered  totally  incapable  of  reflection 
at  another  surface  in  certain  definite  positions; 
while,  in  other  positions,  it  will  be  completely  re- 
flected by  the  second  surface.  It  will  be  found  also 
to  have  lost  the  property  of  penetrating  transparent 
bodies  in  particular  positions,  but,  in  others,  to  be 
freely  transmitted  by  them.  Light  thus  modified 
so  as  to  be  incapable  of  reflection  and  transmission 


298  TREATISE   ON   COMETS. 

in  certain  directions,  is  said  to  be  polarized.  On 
this  principle  Arago  proposed  a  photometrical 
method  of  determining  whether  the  comets  are 
self-luminous,  or  shine  by  means  of  the  light  of  the 
sun  reflected  from  their  surface,  and,  perhaps,  even 
their  internal  parts.  This  method  was  applied  at 
the  last  return  of  Halley's  comet,  and  it  was  found 
that  the  apparatus  gave  two  images  presenting  the 
complementary  colors,  one  of  them  being  red  and 
the  other  green.  By  turning  the  instrument  half 
round  the  red  image  became  green,  and  the  reverse. 
It  was  therefore  concluded  that  the  light  of  the 
comet,  or  at  least  the  whole  of  it,  was  not  composed 
of  rays  possessing  the  property  of  direct  light. 
The  same  experiment's  were  repeated  with  this 
comet  by  different  observers,  and  always  with  the 
same  result.  Similar  experiments  have  subsequently 
been  made  on  some  of  the  brightest  comets,  and  it 
is  found  that  their  light  exhibits  precisely  the  same 
phenomena.  This  shows  that  the  light  of  comets 
must  be  partly  composed  of  that  received  from  the 
sun,  even  if  they  have  a  light  of  their  own.  All 
that  can  be  proved  by  the  polarization  of  their  light, 
is  that  they  may  possibly  shine  wholly  by  reflection, 
while  the  considerations  previously  adduced  are 
sufficient  to  establish  the  possibility  thus  admitted 
as  a  physical  fact,  In  view  of  all  these  facts  it 
seems  reasonable  to  conclude,  without  hesitation, 


TAILS   OF   COMETS.  299 

that  the  comets  shine  by  means  of  the  light  which 
they  receive  from  the  sun,  reflected  from  all  their 
parts. 

The  next  important  consideration  connected  with 
the  theory  of  the  physical  constitution  of  comets,  is 
that  which  relates  to  the  formation  and  develop- 
ment of  their  tails.  The  idea  of  a  tail  or  train  of 
vast  extent  is,  in  the  popular  mind,  inseparably  con- 
nected with  the  physical  appearance  of  these  won- 
derful bodies ;  but  it  should  be  remarked,  that  by 
far  the  great  majority  of  comets  are  not  thus 
attended.  Those  which  have  been  so  brilliant  as 
to  be  visible  to  the  naked  eye,  have  usually  exhibited 
the  phenomenon  of  a  train  of  fainter  light,  extend- 
ing in  a  direction  nearly  opposite  the  sun.  The 
direction  of  the  tail  is,  however,  by  no  means  inva- 
riable. Sometimes  it  has  happened  that  it  formed 
a  considerable  angle  with  a  line  drawn  to  the  sun, 
and  cases  have  occurred  in  which  it  was  actually  at 
right  angles  to  it.  It  has  also  been  observed  that 
the  tails  of  comets,  as  a  general  result,  incline  con- 
stantly toward  the  region  from  which  the  comet  is 
moving,  as  if,  in  its  progress  through  space,  it  were 
subject  to  the  retarding  influence  of  some  resisting 
medium,  its  nebulous  atmosphere  being,  on  account 
of  the  resistance  thus  imposed,  compelled  to  remain 
behind  the  solid  nucleus  in  the  form  of  a  tail. 

We  have  already,  in  the  descriptions  of  some  of 


300  TREATISE   ON   COMETS. 

the  most  remarkable  comets  given  in  the  preceding 
pages,  had  occasion  to  allude  to  the  curved  form 
and  absolute  dimensions  of  the  tails  of  comets,  and 
have  also  noticed  the  various  other  characteristics 
by  which  these  somewhat  remarkable  appendages 
are  distinguished.  It  is  proposed,  therefore,  in  this 
connection,  to  give  simply  some  of  the  various  the- 
ories which  have  hitherto  been  advanced  in  expla- 
nation of  this  phenomenon.  It  will,  of  course,  be 
useless  to  attempt  to  give  all  the  theories  which 
human  ingenuity  has  devised ;  and  we  shall,  there- 
fore, explain  only  those  which  have  been  received 
with  the  greatest  favor,  before  proceeding  to  illus- 
trate that  which  is  now  satisfactorily  ascertained  to 
be  the  true  theory  of  the  development  of  these 
anomalous  appendages. 

The  more  ancient  theory  of  the  fprmation  of  the 
tails  of  comets,  in  which  they  are  supposed  to  be 
formed  by  the  lighter  parts  being  thrown  off  by  the 
resistance  of  the  ether  through  which  the  comet 
passed,  has  just  been  alluded  to.  In  more  modern 
times  the  prevailing  opinion  has  been,  that  these 
particles  are  driven  off  by  the  impulsive  force  of  the 
sun's  rays.  The  fallacy  of  the  former  opinion  is 
evident  from  the  fact  that  the  tails  are  generally 
directed  from  the  sun ;  and  that,  whatever  the  rela- 
tive direction  may  be,  it  remains  very  nearly  the 
same  both  before  and  after  the  perihelion  passage. 


TAILS   OF   COMETS.  301 

The  more  modern  theory  may  be  supported  with 
considerable  force  of  reasoning;  yet  the  latest  in- 
vestigations have  indicated  beyond  a  doubt  the  true 
theory  of  the  formation  of  the  tail.  Another  theory 
which  has  been  advanced  supposes  the  tail  to  be 
formed  by  the  sun's  rays  slightly  refracted  by  the 
nucleus  in  traversing  the  envelope  of  the  comet, 
and  uniting  in  an  infinite  number  of  points  beyond 
it,  throwing  a  stronger  than  ordinary  light  on  the 
ethereal  medium,  near  to  or  more  remote  from  the 
comet,  as  the  ray  is  more  or  less  refracted  from  its 
relative  position  and  direction. 

The  phenomena  presented  by  Halley's  comet  at 
its  last  appearance,  when  considered  in  detail,  led 
Sir  John  Herschel  to  conclude:  1st.  "That  the 
matter  of  the  nucleus  of  a  comet  is  powerfully  ex- 
cited and  dilated  into  a  vaporous  state  by  the  action 
of  the  sun's  rays,  escaping  in  streams  and  jets  at 
those  points  of  its  surface  which  oppose  the  least 
resistance,  and  in  all  probability  throwing  that  sur- 
face, or  the  nucleus  itself,  into  irregular  motions 
by  its  reaction  in  the  act  of  so  escaping,  and  thus 
altering  its  direction.  2d.  That  this  process  chiefly 
takes  place  in  that  portion  of  the  nucleus  which  is 
turned  toward  the  sun  —  the  vapor  escaping  chieflv 
in  that  direction.  3d.  That  when  so  emitted,  it  is 
prevented  from  proceeding  in  the  direction  origin- 
ally impressed  upon  it,  by  some  force  directed  from 
26 


TREATISE   ON    COMETS. 

the  sun,  drifting  it  back,  and  carrying  it  out  to  vast 
distances  behind  the  nucleus,  forming  the  tail,  or  so 
much  of  the  tail  as  can  be  considered  as  consisting 
of  material  substance.  4th.  That  this  force,  what- 
ever its  nature,  acts  unequally  on  the  materials  of 
the  comet,  the  greater  portion  remaining  unva- 
porized,  and  a  considerable  part  of  the  vapor  actu- 
ally produced  remaining  in  its  neighborhood,  form- 
ing the  head  and  coma.  5th.  That  the  force  thus 
acting  on  the  materials  of  the  tail  cannot  possibly 
be  identical  with  the  ordinary  gravitation  of  matter, 
being  centrifugal  or  repulsive,  as  respects  the  sun, 
and  of  an  energy  very  far  exceeding  the  gravitating 
force  toward  that  luminary.  This  will  be  evident 
if  we  consider  the  enormous  velocity  with  which 
the  matter  of  the  tail  is  carried  backward,  in  oppo- 
sition both  to  the  motion  which  it  had  as  part  of  the 
nucleus,  and  to  that  which  it  acquired  in  the  act  of 
its  emission,  both  which  motions  have  to  be  de- 
stroyed in  the  first  instance,  before  any  movement 
in  the  contrary  direction  can  be  impressed.  6th. 
That  unless  the  matter  of  the  tail  thus  repelled  from 
the  sun  be  retained  by  a  peculiar  and  highly  ener- 
getic attraction  to  the  nucleus,  differing  from  and 
exceptional  to  the  ordinary  power  of  gravitation,  it 
must  leave  the  nucleus  altogether ;  being  in  effect 
carried  far  beyond  the  coercive  power  of  so  feeble 
a  gravitating  force  as  would  correspond  to  the 


TAILS   OF   COMETS.  303 

minute  mass  of  the  nucleus;  and  it  is  therefore 
very  conceivable  that  a  comet  may  lose,  at  every 
approach  to  the  sun,  a  portion  of  that  peculiar 
matter,  whatever  it  be,  on  which  the  production  of 
its  tail  depends,  the  remainder  being  of  course  less 
excitable  by  the  solar  action,  and  more  impassive 
to  his  rays,  and  therefore  more  nearly  approximating 
to  the  nature  of  the  planetary  bodies." 

The  repulsion  which  is  thus  assumed  to  be  exer- 
cised by  the  sun,  was  exhibited  by  Donati's  comet 
in  the  most  palpable  manner.  The  question  may 
now  arise  whether  this  force  is  real  or  apparent; 
whether,  if  real,  it  is  a  polar  force,  like  magnetism 
and  electricity,  or  a  simple  force  like  that  of  gravity; 
and  whether,  if  apparent,  it  results  simply  from  the 
difference  of  action  of  the  solar  attraction  on  the 
various  parts  of  the  comet,  or  from  its  different 
action  on  the  molecules  of  a  gravitating  ether,  and 
upon  those  of  the  comet.  There  are  thus  exhibited 
four  cases  to  be  considered,  two  of  which,  however, 
may  hardly  with  propriety  be  regarded  as  strictly 
connected  with  a  repulsive  force,  such  as  will  be 
proved  to  exist.  The  idea  of  an  apparent  repulsion 
is  to  assimilate  the  phenomena  of  the  comets  to 
that  of  the  seas,  only  on  a  vastly  greater  scale.  The 
sun  is  supposed  to  act  in  two  ways ;  first,  by  its 
attraction,  or  by  its  gravitation,  and  then  by  its 
heat.  The  attraction  of  the  sun  on  a  solid  body 

I 


304  TREATISE   ON   COMETS. 

surrounded  by  a  nebulous  or  fluid  atmosphere  would 
necessarily  change  its  figure  from  the  spherical  form 
to  an  oblong  or  ellipsoidal  form,  in  case  the  exterior 
or  disturbing  force  is  very  small  in  comparison  with 
the  central  gravity  of  the  body,  and  its  diameter 
very  small  in  comparison  with  the  distance  of  the 
source  of  the  disturbance.  In  this  way  the  attraction 
of  the  sun  and  moon  produces  the  ebbing  and  flow- 
ing of  the  tides  of  the  sea,  and  of  the  atmosphere. 
In  the  case  of  a  comet  —  the  mass  being  small,  and 
the  volume  extended  —  it  is  evident  that  the  solar 
attraction  can,  at  a  certain  distance,  be  much  greater 
than  that  which  exists  between  the  different  parts 
of  the  comet ;  and  also,  that  this  attraction  will  be 
very  different  in  various  parts  of  its  volume.  In 
order,  therefore,  to  determine  the  figure  of  a  comet 
at  a  given  distance  from  the  sun,  it  is  only  neces- 
sary to  find  the  form  which  a  fluid  mass  will  assume, 
when  the  attraction  of  the  sun  is  comparable  with 
or  even  greater  than  the  gravitation  between  its 
parts,  its  volume  being  considered  as  very  great  in 
comparison  with  its  distance  from  him.  Under  these 
conditions  the  law  of  equilibrium  of  a  moving  mass 
has  been  determined,  whose  physical  constitution 
is  supposed  to  be  a  nucleus  of  solid  matter  sur- 
rounded by  concentric  layers  of  an  elastic  atmos- 
phere, with  a  density  continually  decreasing  from 
the  centre  outward.  It  should  be  remarked,  how- 


TAILS   OF   COMETS.  305 

ever,  that  no  direct  solution  of  the  question  thus 
presented  has  been  satisfactorily  accomplished ;  and 
it  is  Simply  contended  that,  as  a  necessary  conse- 
quence, the  densest  portion  of  the  comet,  supposed 
to  be  a  fluid  mass,  would  not  remain  at  the  centre, 
but  would,  on  the  contrary,  approach  the  sun ;  and 
that,  in  the  case  of  a  comet  constituted  chiefly  of  an 
extremely  attenuated  vapor,  its  form  would  be 
analogous  to  that  of  a  column  of  expansible  atmos- 
phere, having  its  base  supported  by  the  attracting 
body,  the  denser  layers  being  always  nearest  the 
base.  In  this  way  it  is  supposed  that  the  tail  of  a 
comet  may  be  formed,  resting  in  space,  isolated  and 
extended,  and  always  sensibly  in  a  direction  oppo- 
site to  the  sun.  The  case  of  a  solid  body  surrounded 
by  a  nebulous  atmosphere  is  considered  as  the  limit 
of  a  minimum  change  of  form,  and  that  of  an 
atmospheric  column  as  the  maximum  limit.  Be- 
tween these  an  infinity  of  varied  forms  may  be 
imagined ;  and  hence  this  theory  has  been  urged  as 
wholly  sufficient  to  explain  all  the  phenomena  which 
have  been  observed  in  the  formation  of  the  tails  of 
comets.  The  sudden  and  anomalous  changes  in 
the  vicinity  of  the  nucleus  as  the  comet  approaches 
the  sun,  are  attributed  to  the  influence  of  the  solar 
heat,  which  must  necessarily  produce  an  enormous 
dilatation  of  its  mass.  It  is  further  contended  that 
the  expansion  of  the  comet  by  heat,  and  its  subse- 
26*  u 


306  TREATISE   ON    COMETS. 

quent  condensation  or  contraction  by  cold,  in  con- 
nection with  the  operation  of  the  solar  attraction, 
as  just  explained,  are  sufficient  to  account  for  every 
phenomenon  connected  with  the  structure  of  comets 
which  has  hitherto  been  observed;  and  meteoro- 
logical phenomena  on  our  earth  are  adduced  as 
beautiful  examples  of  similar  effects.  This  theory 
has  been  urged  with  great  force  of  reasoning,  and 
has,  indeed,  been  received  with  considerable  favor; 
yet  it  must  be  admitted  that  a  detailed  examination 
of  the  recorded  phenomena  presented  by  some  of 
the  largest  comets,  shows  conclusively  that  it  is 
inadequate  to  their  complete  explanation. 

A  favorite  theory  of  Newton  was  that  the  molecules 
of  the  circumambient  ether,  overheated  by  means 
of  the  comet,  wilich  absorbs  the  solar  rays,  and 
transmits  the  heat  to  the  ether,  would  become  lighter 
than  its  exterior  layers,  and  would,  therefore,  flow 
backward  in  a  direction  opposite  to  the  sun,  carry- 
ing with  them  a  part  of  the  molecules  of  the  comet. 
In  this  way  it  is  imagined  that  the  phenomena  of 
the  tails  of  comets  may  be  explained  without  resort- 
ing to  a  direct  repulsive  force,  thus  avoiding  com- 
pletely whatever  may  seem  to  complicate  and  con- 
fuse the  beautiful  conception  of  universal  gravita- 
tion, by  the  introduction  of  an  opposing  force.  This 
theory  of  the  action  of  the  radiant  heat  of  the  comet 
on  the  ether  which  surrounds  it,  and  that,  previously 


TAILS   OF   COMETS.  307 

stated,  which  connects  the  formation  of  the  tails  of 
comets  with  the  combined  action  of  the  attraction. of 
the  sun,  and  the  solar  heat  acting  directly  on  the 
molecules  of  the  comet,  are  those  which  have  been 
classed  as  belonging  to  what  may  be  termed  appa- 
rent repulsion. 

Kepler  and  others  have  attributed  to  the  light  of 
the  sun  a  repulsive  action,  sufficient  to  drive  off  the 
lighter  portions  of  a  comet  in  the  form  of  a  tail. 
This  is  the  case  of  a  direct  repulsive  force,  operating 
like  gravity,  only  in  a  contrary  direction.  With 
this  idea  of  repulsion  as  a  basis,  and  omitting  all 
considerations  in  respect  to  the  character  of  light — 
as  not  being  essentially  of  any  account,  in  the  con- 
clusions which  may  be  arrived  at  —  Faye  has 
endeavored  to  develop  a  complete  theory  of  the 
figure  and  physical  constitution  of  comets.  Before 
proceeding,  however,  to  give  an  explanation  of.  this 
theory,  it  may  be  well  to  state  that  Bessel  and 
Olbers  have  attributed  the  formation  of  the  tails 
and  envelopes  of  comets  to  the  operation  of  a  repul- 
sive force  emanating  chiefly  from  the  sun,  and  being, 
like  magnetism  and  electricity,  a  polar  force.  Olbers 
supposed  the  repulsive  agent  to  be  electricity,  and 
imagined  that  it  acted  so  as  to  produce  a  repulsive 
force  in  the  sun,  a  similar  force  in  the  comet  itself, 
and  a  specific  action  of  the  sun  such  as  may  be 
supposed  to  be  exhibited  in  the  simultaneous  exist- 


308  TREATISE   ON    COMETS. 

ence  of  several  distinct  tails.  Bessel  subjected  the 
theory  of  Olbers  to  a  rigorous  mathematical  calcu- 
lation, introducing,  however,  such  modifications  as 
the  exigencies  of  the  case  demanded.  He  supposed 
the  action  of  the  sun  on  a  comet  to  be  exercised  in 
two  different  ways,  namely,  by  a  general  action 
equal  for  every  part,  and  by  a  differential  action, 
strongest  at  those  points  nearest  the  sun,  and  most 
feeble  at  those  parts  most  remote  from  him.  This 
he  conceived  to  be  a  necessary  consequence,  no 
matter  what  the  nature  of  the  force  exercised  may 
be.  Thus,  for  example,  in  the  case  of  the  force  of 
gravitation,  there  is  a  general  action  which  produces 
the  motion  of  the  centre  of  gravity,  and  a  differ- 
ential action  which  causes  a  motion  of  rotation  ;  the 
latter  being  always  feebler  than  the  former.  He, 
therefore,  supposes  that  the  comet  experiences  at 
first  the  general  action  of  the  sun,  and  that,  under 
this  action,  it  emits  from  every  part  of  its  nucleus 
particles  of  matter  whose  polarity  is  negative  with 
reference  to  the  sun.  On  approaching  its  perihelion, 
the  comet  will  experience,  finally,  the  second  mode 
of  action,  and  this  he  imagines  to  give  to  it  the  two 
poles,  the  positive  pole,  with  reference  to  the  sun, 
being  on  the  side  nearest  that  luminary.  At  this 
point  an  emission  of  nebulous  matter  will  com- 
mence ;  and,  since  the  polarity  is  positive,  this  emission 
will  be  directed  toward  the  sun.  In  passing  through 


TAILS    OF   COMETS.  309 

the  nebulous  atmosphere  which  is  aiready  filled 
with  negative  particles,  and  which  is  continually 
forming  anew,  the  nebulous  matter  emitted  will 
lose,  by  degrees,  its  primitive  polarity,  and  even 
assume,  finally,  the  opposite  polarity,  before  it 
reaches  the  limit  of  this  atmosphere.  This  emission 
from  the  nucleus  will  now  be  repelled  by  the  sun, 
and  driven  back  to  form  the  tail.  This  is  a  general 
statement  of  Bessel's  theory  as  modified  from  that 
of  Olbers. 

The  phenomenon  of  the  regular  formation  of 
luminous  sectors,  of  which  the  great  comet  of  1811 
furnished  so  beautiful  an  example,  led  Olbers  to 
conclude,  without  hesitation,  that  the  hypothesis 
made  by  Kepler  and  Euler,  which  we  have  already 
stated,  was  necessarily  untenable,  since  the  solar 
repulsion  could  not  produce  an  emission  directed 
from  the  nucleus  itself  toward  the  sun.  It  was  for 
this  reason  that  he  considered  it  necessary  to  endow 
both  the  comet  and  the  sun  with  repulsive  forces, 
which  he  attributed  to  a  common  cause,  namely,  to 
the  electricity  which  might  be  supposed  to  be  de- 
veloped by  the  approach  of  the  comet  to  the  sun. 
The  periodic,  or  nearly  periodic  fluctuations  of  the 
luminous  sectors  exhibited  in  the  case  of  Halley's 
comet,  at  its  last  return,  led  Bessel  to  assume  the 
existence  of  a  polar  force  developed  in  the  comet 
under  the  influence  of  the  sun,  and  in  this  way  to 


310  TREATISE   ON   COMETS. 

attempt  to  reconcile  all  the  varied  phenomena 
which  have  been  observed.  Faye,  adopting  the 
fundamental  idea  of  a  repulsive  force,  the  existence 
of  which  he  considers  to  be  conclusively  established, 
attempts  to  explain  not  only  the  figure  of  the  comets, 
but  also  the  acceleration  of  their  motion,  which  we 
have  stated  to  be  due  to  the  resistance  of  an  ethereal 
fluid  pervading  all  space.  He  attributes  to  the 
force  of  repulsion  a  mode  of  propagation,  not  in- 
stantaneous like  that  of  gravitation,  but  in  regular 
succession,  like  the  undulations  of  the  ether  in  pro- 
ducing light.  He  supposes  the  repulsive  force  to 
be  resolved  into  two  component  parts,  one  acting 
in  the  direction  of  the  line  joining  the  centre  of  the 
sun  with  that  of  the  comet,  and  the  other  in  the 
direction  of  a  line  tangent  to  the  orbit  of  the  comet 
at  the  point  where  it  is  situated.  The  first  com- 
ponent force  may  be  supposed  to  determine  the 
figure  of  the  comets,  in  chiding  the  formation  of 
the  tail,  while  the  second  may  act  to  produce  the 
acceleration  of  their  motions.  The  repulsive  force 
being  supposed  to  be  propagated  by  a  series  of  suc- 
cessive undulations,  it  follows  that  it  will  not  depend 
directly  on  the  mass  of  the  sun,  but  on  the  extent 
of  his  surface  and  its  physical  condition.  The  in- 
tensity of  its  action  on  any  body  whatever  will  vary 
inversely  as  the  square  of  its  distance.  In  this  way, 
Faye  considers  it  possible  to  give  n  most  simple  and 


TAII<S   OF     COMETS.  311 

natural  explanation  of  the  phenomena  which  led  to 
the  hypotheses  of  Gibers  and  Bessel,  thus  avoiding 
a  metaphysical  objection,  which  he  contends  may 
be  justly  urged  against  the  existence  of  a  force  such 
as  these  astronomers  have  assumed.  The  objection 
referred  to  is  that  these  hypotheses  attribute  simul- 
taneously to  the  same  matter  a  repulsive  force  and 
an  attractive  force  of  the  same  nature,  the  same 
direction,  and  the  same  law,  with  the  exception 
only  of  the  specific  and  limited  operation  of  the 
former,  which  distinguishes  it  from  the  universality 
of  the  latter. 

In  further  confirmation  of  his  theory,  Faye  adduces 
the  fact  that  the  theories  of  Olbers  and  Bessel  seem 
to  present  almost  insuperable  difficulties  in  the 
explanation  of  the  phenomena,  which  completely 
vanish  when  we  adopt  the  simple  and  unique  theory 
of  Kepler,  which  he  has  adopted  as  the  basis  of  his 
own.  He  regards  the  modifications  of  the  original 
theory  of  Olbers  introduced  by  Bessel  as  extremely 
complicated,  in  which  it  is  supposed  that,  under  the 
influence  of  the  sun,  the  nucleus  acquires  polarity, 
and  emits,  in  the  direction  of  the  sun,  particles  which 
are  negatively  electrified,  while,  at  the  same  time, 
the  sun  exercises,  on  this  part  of  the  comet,  a 
positive  action.  He  regards  it  as  an  important 
objection  to  Bessel's  theory,  that,  in  order  to  explain 
the  manner  in  which  these  particles  or  molecules 


312  TREATISE   ON   COMETS. 

of  nebulous  matter,  notwithstanding  their  negative 
polarity,  cease  to  be  attracted  by  the  sun,  and  finally 
become  subject  to  an  energetic  repulsion  on  the 
part  of  the  sun,  which  drives  them  back  from  him, 
it  becomes  necessary  to  suppose  that  by  virtue  of  a 
general  action  of  the  sun  —  while  the  comet  is  yet 
far  distant,  and  before  the  development  of  the 
polarity  —  the  part  of  the  comet  from  which  the 
emission  takes  place  has  been  positively  polarized 
like  the  sun.  He  also  cites  the  fact  that  Bessel 
admitted  that  the  molecules  which  were  thus  differ- 
ently polarized  will  become  neutralized,  and  that 
the  particles  emitted  by  the  nucleus  finally  lose  their 
negative  polarity,  and  assume  a  positive  one,  even 
before  they  are  very  distant  from  the  nucleus,  and 
ultimately  become  subject  to  the  repulsive  influence 
of  the  sun,  being  driven  back  to  form  the  tail.  The 
intensity  of  this  force  of  repulsion  will,  it  is  urged, 
vary  with  the  nature  of  these  molecules.  In  the 
case  of  Halley's  comet,  at  its  last  return,  its  mole- 
cules were  repelled  by  the  sun  by  a  force  nearly 
twice  as  great  as  that  of  attraction ;  and  those  of 
Donati's  comet  (1858)  by  a  force  whose  intensity 
was  a  little  more  than  one-third  that  of  gravity  in 
the  case  of  the  principal  tail,  and  by  a  force  nearly 
seventeen  times  greater  in  the  case  of  the  secondary 
train.  The  difficulty  which  Faye  thinks  very  im- 
portant, is  to  explain  why  the  sun  should  exercise 


TAILS   OF   COMETS.  313 

on  the  different  particles  of  the  comet  two  separate 
and  distinct  repulsive  forces  of  such  unequal  in- 
tensity. This,  however,  has  been  explained  by 
supposing  that  different  parts  of  the  comet  had  a 
vastly  different  specific  gravity,  being  actually  lighter 
than  the  surrounding  ether,  and  that  these  particles 
are  thus  repelled  through  the  ether  with  very  differ- 
ent velocities  —  which  was  essentially  the  idea  of 
Newton. 

Soon  after  the  appearance  of  the  great  comet  of 
1843,  Professor  Norton  proposed  a  theory  of  the 
formation  of  the  tails  of  comets  which  in  some 
respects  resembled  the  theories  of  Bessel  and  Olbers. 
He  was  led  to  the  final  hypothesis  which  he  formed 
by  such  considerations  as  the  following,  having  been 
entirely  ignorant  of  the  fact  that  in  some  points  of 
his  theory  he  had  been  anticipated  by  others.  1st. 
The  general  direction  and  situation  of  the  tail  with 
respect  to  the  sun,  show  that  this  luminary  is  con- 
cerned, directly  or  indirectly,  in  its  formation,  while 
the  changes  which  take  place  in  the  dimensions  and 
figure  of  the  comet,  both  in  approaching  its  peri- 
helion, and  in  receding  from  it,  lead  to  the  same 
inference.  2d.  Since  the  tail  lies  in  the  direction 
very  nearly  of  a  line  drawn  from  the  centre  of  the 
sun  through  that  of  the  comet,  the  particles  of 
matter  of  which  it  is  composed  must  have  been 
driven  off  from  the  head  by  some  force  exerted  in  a 
27 


314  TREATISE   ON    COMETS. 

direction  from  the  sun.  3d.  This  force  cannot 
emanate  from  the  nucleus,  for  such  a  force  would 
expel  the  nebulous  matter  surrounding  the  nucleus 
in  all  directions,  instead  of  one  direction  only.  This 
objection,  however,  might  be  removed  by  supposing 
that  a  repulsive  action  is  exerted  by  the  nucleus 
only  from  that  side  which  is  most  remote  from  the 
sun ;  yet  it  may  be  perceived  that  such  a  force  as 
this  would  not  give  to  the  tail  the  form  and  direction 
which  observations  indicate,  when  it  is  considered 
that  the  tail  has  its  origin  in  the  nebulosity  at  the 
side  of  the  nucleus.  It  is  thus  evident  that  no  one 
force,  having  its  origin  in  the  head  of  the  comet, 
can  be  conceived  of  which  would  be  adequate  to 
the  production  of  the  tail.  There  is,  however,  a 
possibility  that  a  repulsive  action  of  the  nucleus  on 
the  matter  of  the  nebulosity  may  be  combined  wTith 
some  other  force  foreign  to  the  comet,  as  an  auxiliary 
cause  in  producing  the  phenomena  of  the  tail ;  — 
its  effects  on  the  side  toward  the  sun  being  con- 
tracted, at  a  definite  distance  from  the  nucleus,  by 
this  latter  force.  4th.  It  seems  to  follow  from  what 
has  just  been  stated,  as  a  necessary  consequence, 
that  the  matter  of  the  tail  is  driven  off  from  the 
head  by  some  force  foreign  to  the  comet,  and  taking 
effect  from  the  sun  outward.  5th.  This  force,  what- 
ever its  nature  may  be,  must  be  supposed  to  extend 
far  beyond  the  orbit  of  the  earth ;  and  although 


TAILS   OF   COMETS.  315 

nothing  can  be  predicated  with  certainty  respecting 
the  law  of  its  variation,  yet  it  is  at  least  probable 
that,  like  all  known  central  forces,  it  varies  inversely 
as  the  square  of  the  distance. 

From  such  considerations  as  these  Korton  was 
led  to  the  conclusion  that  there  existed  a  repulsive 
force  in  the  sun,  and  that  although  it  might  operate 
so  as  to  drive  off  the  nebulous  matter  to  greater 
and  greater  distances,  without  destroying  the  con- 
nection of  the  parts — the  head  and  tail  thus  revolving 
as  one  connected  mass  —  or,  so  as  continually  to 
detach  portions  of  the  nebulosity,  and  repel  them 
to  an  indefinite  distance  into  free  space ;  yet  that  it 
was  most  probable  that  the  latter  mode  of  action 
was  the  one  which  actually  exists.  After  a  long 
train  of  reasoning  based  on  the  phenomena  of  the 
comets  which  had  hitherto  appeared  —  but  which  it 
will  be  unnecessary  to  give  in  this  connection  —  he 
arrived  finally  at  the  conclusion  that  the  tail  of  a 
comet  is  made  up  of  particles  of  matter  continually 
flowing  away,  at  a  very  rapid  rate,  from  the  head 
into  free  space,  and  that  at  any  one  instant  we  see 
the  collection  of  all  the  particles  that  have  been 
emitted  during  a  certain  previous  interval.  Accord- 
ing to  this  hypothesis,  at  the  end  of  any  such 
interval  we  are  looking  at  an  entirely  new  tail.  The 
particles  may  be  supposed  to  be  detached  from  the 
outer  portions  of  the  nebulosity  of  the  comet,  by 


316  TREATISE   ON   COMETS. 

the  repulsive  force  of  the  sun ;  in  which  case  they 
would  fly  off  in  the  directions  of  the  lines  diverging 
from  the  sun,  along  which  the  force  acts,  hut  would 
be  made  hy  the  attraction  of  the  nucleus  to  pursue 
paths  slightly  deviating  from  these  lines  and  con- 
cave toward  the  axis  of  the  tail ;  or,  as  Bessel  and 
Olbers  have  supposed,  they  may  undergo  some 
modification  hy  the  action  of  the  sun,  by  reason  of 
which  they  are  first  repelled  outward  from  the 
nucleus,  and  then  driven  away  from  the  sun,  into 
the  depths  of  space,  by  virtue  of  his  superior  repul- 
sion. According  to  the  latter  supposition,  they 
acquire  an  initial  velocity  in  leaving  the  nucleus, 
and  subsequently,  under  the  action  of  the  sun's 
repulsive  force,  they  will  move  off  in  hyperbolas, 
having  the  sun  in  their  remote  focus,  and  concave 
toward  the  axis  of  the  tail. 

By  means  of  this  theory  Norton  was  enabled  to 
explain  the  curved  form  of  the  tail,  and  the  general 
fact  of  its  deviation  from  the  position  of  direct 
opposition  to  the  sun ;  and  also  to  indicate  the 
causes  in  operation  to  produce  the  various  degrees 
of  curvature,  and  various  positions  of  the  tail, 
noticed  in  the  case  of  different  comets,  and  in 
different  positions  of  the  same  comet  in  its  orbit. 
The  appearance  of  Donati's  comet  in  1858  induced 
him  to  modify  his  theory  in  some  particulars,  and 
to  reduce  it  to  a  complete  and  systematic  form.  The 


TAILS   OF   COMETS.  317 

phenomena  of  successive  envelopes  presented  by 
this  comet  to  the  most  remarkable  degree,  led  him 
to  conclude  that  the  nebulous  envelope  of  the  head 
of  a  comet  was  probably  in  the  same  dynamical  con- 
dition with  the  tail ;  that  the  cometic  matter  was,  in 
the  first  instance,  expelled  from  the  nucleus  on  the 
side  toward  the  sun,  and  then  driven  back  to  form  the 
tail,  by  virtue  of  the  repulsive  force  emanating  from 
the  sun;  and  that  this  process  went  on  continuously; 
the  nebulous  matter  rising  perpetually,  like  a  gush- 
ing fountain  of  light,  and  the  luminous  jets  bending 
back,  in  graceful  curves,  before  the  repelling  energy 
of  the  sun.  He,  therefore,  regarded  the  nebulous 
envelope  of  the  head,  and  the  tail  of  a  comet,  as 
simply  different  parts  of  the  same  flowing  stream ; 
and  imagined  that  the  cometary  matter  was,  in  the 
first  place,  urged  away  from  the  nucleus  by  an 
incessant  force  of  repulsion  exerted  by  its  mass,  and 
that  this  force  and  the  repulsive  action  of  the"  sun, 
like  all  radiant  actions,  varied  inversely  as  the  square 
of  the  distance.  Other  suppositions  were  also  made 
—  as  that  the  expelling  action  was  an  instantaneous 
repulsive  force  ;  and  again,  that  the  nebulous  matter 
was  first  detached  by  a  superficial  force  of  projection, 
and  subsequently  acted  on  by  the  repulsion  of  the 
mass  of  the  nucleus.  By  means  of  this  theory, 
Professor  Norton  was  enabled  to  explain  all  the 
phenomena  presented  by  the  great  comet  of  1858, 
27* 


318  TREATISE   ON    COMETS. 

and  also  to  derive  an  approximate  value  of  the  density 
of  the  matter  composing  the  nucleus  of  the  comet. 
He  found  that  the  nucleus  of  this  comet  was  of  from 
five  to  six  times  the  density  of  water,  or  about  the 
same  as  the  mean  density  of  the  earth.  He  per- 
formed similar  calculations  in  the  case  of  four  other 
comets,  and  found  that  their  density  is  from  four  to 
fourteen  times  that  of  water.  These  results  may 
seem  to  be  at  variance  with  the  generally  received 
opinion  in  regard  to  the  physical  constitution  of 
these  bodies ;  and  it  must  be  admitted  that  they  are 
at  most  but  rude  approximations;  —  yet,  he  con- 
sidered them  as  affording  sure  indications  that  the 
telescopic  nucleus  of  a  bright  comet  is  not  entirely 
gaseous,  or  nebulous,  but  that  it  is  either  liquid  or 
solid,  or  composed  of  both  liquid  and  solid  matter, 
like  the  earth. 

He  accounted  for  the  rise  and  gradual  recess  of  a 
succession  of  envelopes  from  the  nucleus,  by  sup- 
posing that  the  evolution  of  cometic  matter  first 
begins  at  the  polar  regions  of  the  nucleus,  and 
gradually  extends  toward  the  equatorial  parts,  the 
comet  being  supposed  to  rotate  about  an  axis  per- 
pendicular to  the  plane  of  its  orbit.  Under  these 
circumstances,  if  the  comet  is  observed  under  a 
large  angle  to  the  plane  of  its  orbit,  as  was  the  case 
with  Donati's  comet,  after  the  perihelion  passage, 
the  outline  of  the  visible  envelope  would  steadily 


TAILS   OF   COMETS.  319 

enlarge,  as  the  process  of  evolution  was  propagated 
farther  and  further  from  the  polar  regions  of  the 
nucleus.  In  this  explanation,  he  supposes  that  the 
envelopes  are  not  spherical  in  form,  but  that  each 
approximates  in  its  form  to  that  of  a  flat  semicir- 
cular disc,  of  a  definite  thickness.  The  dark  stripe 
extending  from  the  nucleus  to  a  considerable  dis- 
tance in  the  tail,  which  we  have  already  noticed  in 
the  description  of  Donati's  comet,  is  supposed  to 
show  conclusively  that  the  envelopes  from  which 
the  tail  proceeded  were  much  flattened  in  the 
direction  of  the  line  of  sight ;  or  approximately  in 
that  direction. 

Such  are  the  principal  theories  which  have  been 
devised,  in  order  to  explain  all  the  anomalous 
appearances  presented  by  the  comets ;  and  a  careful 
consideration  of  these  different  hypotheses  in  con- 
nection with  observed  facts,  will  make  it  evident 
that  there  exists  in  nature  a  general  force  of  repul- 
sion, exerted  by  all  masses,  and  operating  tinder 
certain  circumstances  upon  matter  in  an  extremely 
attenuated  state.  The  idea  of  repulsion  being  thus 
established  on  a  firm  basis,  as  a  physical  fact,  it  is 
only  necessary  to  determine  which  of  the  various 
theories  of  the  mode  of  operation  of  the  repulsive 
force  will  best  explain  all  the  varied  phenomena  of 
cornets  in  general.  The  hypotheses  of  Bessel,  Olbers, 
and  Norton,  are  essentially  the  same,  and  differ  only 


320  TREATISE   ON    COMETS. 

in  certain  supposed  peculiarities  of  the  force  of 
repulsion,  and  in  its  mode  of  operation.  Bessel  was 
the  first  to  apply  a  vigorous  mathematical  analysis 
in  the  determination  of  the  effects  of  such  a  force 
as  is  found  to  exist.  The  modifications  which  he 
found  it  necessary  to  introduce  into  Gibers'  theory, 
in  order  to  explain  the  phenomena  observed  in  the 
case  of  Halley's  comet,  at  its  last  return,  have  been 
objected  to  as  rendering  the  results  contradictory 
when  an  attempt  is  made  to  derive  the  intensity  of 
the  forces  in  operation.  The  appearance  of  the 
great  comet  of  1858  under  circumstances  peculiarly 
favorable  to  the  final  determination  of  the  theory 
of  the  formation  of  the  tail  by  means  of  a  repulsive 
force  common  to  all  bodies,  required  simply  that 
the  various  theories  advanced  should  be  subjected 
to  the  test  of  exact  mathematical  calculation.  This 
was  performed  by  Professor  Pierce  in  the  case  of 
Gibers'  hypothesis,  and  the  results  obtained  were  in 
such  strict  accordance  with  the  observations,  that 
this  theory  is  now  regarded  as  being  established  on 
the  surest  possible  foundation. 

There  is  an  inference  which  may  be  drawn  from 
an  examination  of  the  theory  of  the  operation  of  a 
repulsive  force  in  the  formation  of  the  tail  of  a 
comet,  which  is,  that  some  portions,  at  least,  of  the 
nebulous  matter  will  be  detached  entirely  from  the 
comet,  and  driven  off  into  space.  Under  this  view 


TAILS   OP   COMETS.  321 

it  may  be  supposed  that  the  comets  are  gradually 
wasting  away,  and  will  finally  be  extinguished. 
Observations,  however,  do  not  by  any  means  render 
it  certain  that  these  bodies  are  thus  affected,  and 
the  necessity  of  such  a  supposition  may  perhaps  be 
obviated  by  regarding  the  ethereal  fluid  which  per- 
vades all  space  as  of  sufficient  density  to  afford  a 
sensible  resistance  to  the  flow  of  the  nebulous  matter 
from  the  head  of  the  comet,  thus  limiting  the  extent 
of  the  tail.  There  are,  indeed,  questions  here  pre- 
sented which  we  are  unable  to  answer  satisfactorily, 
and  which  require  a  long  series  of  observations  of 
the  successive  returns  of  a  comet,  for  their  elucida- 
tion. It  may  be  that  the  comets  are  thus  wasting 
away,  but  that  in  their  wanderings  through  endless 
space  they  casually  assimilate  new  cometic  matter 
from  the  waste  of  other  comets,  and  thus  undergo  a 
series  of  perpetual  changes.  It  may  also  happen 
that  two  different  comets  may  approach  each  other 
near  enough  to  cause  their  separate  masses  to 
coalesce,  thus  furnishing  abundant  material  for 
future  losses.  These  and  similar  questions  will 
undoubtedly  be  finally  solved,  though  the  observa- 
tions hitherto  recorded  fail  to  give  a  satisfactory 
answer. 

Another  consideration  which  may  appear  in  this 
connection,  is,  whether  the  tail  of  a  comet,  being 
composed  of  attenuated  nebulous  matter,  can  offer 


322  TREATISE   ON    COMETS. 

any  sensible  resistance  to  the  regular  motion  of  the 
centre  of  gravity  of  the  nucleus.  In  observing  these 
bodies,  their  places  on  the  celestial  vault  are  deter- 
mined under  the  supposition  that  the  point  of 
greatest  brilliancy  corresponds  to  the  centre  of 
gravity  of  the  comet.  The  elements  of  the  orbit 
are  computed  under  this  hypothesis,  and  it  might 
De  expected  that  the  predicted  places  would  not  be 
found  to  correspond  with  the  actual  observations. 
In  the  case  of  all  the  comets,  this  is  to  a  greater  or 
less  extent  true,  but  the  deviations  have  usually 
been  attributed  to  the  necessary  uncertainty  of  the 
observations.  In  the  case,  however,  of  Donati's 
comet,  it  is  found  that  it  is  absolutely  impossible  to 
represent  the  entire  series  of  observations  by  a 
purely  elliptic  orbit,  and  that  the  discrepancies 
between  computation  and  observation  are  such  as 
to  indicate  beyond  a  doubt  that  the  motion  of  the 
nucleus  in  its  orbit  is  sensibly  affected  by  the  tail, 
or  by  the  joint  action  of  this  and  the  repulsive  forces. 
The  question  may  also  arise  as  to  the  nature  or 
cause  of  the  repulsive  force  whose  existence  is  thus 
so  clearly  established.  The  near  approach  of  a 
comet  to  the  sun,  under  an  ever-varying  tempera- 
ture, would  necessarily  produce,  when  near  the  sun, 
the  most  intense  electrical  excitement ;  and  we  may 
thus  infer  that  the  force  partakes  in  some  degree  of 
the  nature  of  electrical  action.  The  existence  of 


TAILS   OF   COMETS.  323 

an  ethereal  fluid  pervading  space  is  beyond  all 
doubt,  and  it  is  certainly  known  that  light  is  pro- 
duced by  certain  vibrations  or  pulsations,  through 
this  ether,  an  effect  which  is  imparted,  to  a  greater 
or  less  extent,  by  all  material  bodies.  The  develop- 
ment of  electric  currents,  as  well  by  magnetic  as 
by  electrical  action,  the  similarity  in  their  mode  of 
action  in  a  great  variety  of  circumstances,  and 
especially  the  production  of  the  spark  from  a  magnet, 
the  ignition  of  metallic  wires,  and  chemical  decom- 
position, all  tend  to  show  that  magnetism  and  elec- 
tricity are  to  be  regarded  as  identical.  The  evolu- 
tion of  light  and  heat  during  the  passage  of  the 
electric  fluid,  the  development  of  electricity  by  heat, 
the  influence  of  heat  on  magnetic  bodies,  and  that 
of  light  on  the  vibration  of  the  compass,  show 
decided  indications  of  some  common  and  invisible 
bond  uniting  these  mysterious  but  powerful  agents 
of  nature.  The  fact  that  these  agencies  operate 
each  with  an  intensity  varying  in  accordance  with 
the  same  law,  is  a  further  proof  of  their  common 
origin.  The  solar  spots,  it  may  likewise  be  re- 
marked, are  found  to  be  connected  intimately  with 
terrestrial  magnetism,  and  in  view  of  all  these  facts, 
it  seems  reasonable  to  conclude  that  light,  heat,  and 
electricity,  are  all  produced  by  successive  undula- 
tions in  the  ethereal  fluid  — just  as  sound  is  caused 
by  vibrations  communicated  to  our  fluid  atmosphere, 


324  TREATISE   ON   COMETS. 

—  yet  under  different  circumstances  with  respect  to 
the  operation  of  the  cause. 

Having   thus    considered   in    detail   the   various 
theories  of  the  physical  constitution  of  these  wan- 
dering bodies,  it  may  not  be  improper  to  add  a  few 
remarks  in  reference  to  their  probable  origin.     La- 
place,  following  the  speculations   of  Sir  William 
Herschel,  applied  the  nebular  theory  of  that  astro- 
nomer to  the  formation  of  the  solar  system,  com- 
prehending the  comets  as  well  as  the  planets  and 
their  satellites.     He  supposes  that  in  the  primeval 
condition  of  the  solar  system,  the  sun  revolved  on 
his  axis,  surrounded  by  an  atmosphere  which,  on 
account  of  an  excessive  heat,  extended  far  beyond 
the  orbit  of  the  remotest  planets.     The  heat  gradu- 
ally diminished,  and  as  the  solar  atmosphere  con- 
tracted by  cooling,  the  rapidity  of  his  rotation  be- 
came increased,  and   the  exterior  portions    of  the 
nebula   by   which   he   was   surrounded,   would   be 
detached  from  the  rest,  the  central  attraction  being 
no  longer  able  to  overcome  the  increased  centrifugal 
force.     This  ring  of  nebulous  matter  would,  from 
various  causes,  be  broken  into  fragments  forming 
a  planet  and  satellites,  or,  which  is  more  probable, 
the  fragments  would  coalesce  into  one  nebular  mass, 
which  would  revolve  around  the  sun  in  an  orbit 
nearly,  if  not  quite  circular,  lying  in  a  plane  nearly 
coincident  with   the  plane   of  the   equator  of  the 


ORIGIN   OF   COMETS.  325 

central  body,  and  revolving  in  its  orbit  in  the  same 
direction  in  which  the  central  globe  rotates  on  its 
axis.  The  same  process  would  again  take  place 
when  the  central  mass  had  again  parted  with  a 
sufficient  quantity  of  heat,  and  thus  the  successive 
planets  may  be  supposed  to  have  been  formed  in  a 
state  of  vapor,  the  process  continuing  until  the 
cohesion  of  the  particles  of  the  central  mass  were 
finally  able  to  resist  any  further  change.  These 
planets,  in  their  turn,  having  each  its  motion  of 
rotation,  would,  as  they  became  gradually  cooled 
and  condensed,  produce  satellites  in  the  same 
manner,  and  by  the  operation  of  the  same  laws,  by 
virtue  of  which  they  were  themselves  formed  from 
the  nebulous  matter  surrounding  the  sun.  It  may 
be  perceived  also  that  the  motions  of  the  satellites 
thus  produced,  and  the  motions  of  rotation  of  the 
planets,  must  be  in  the  same  direction.  Thus  may 
this  strange  and  apparently  fanciful  theory  be  made 
to  account  for  the  most  remarkable  circumstances 
in  the  structure  of  the  solar  system.  The  motions 
of  the  planets  in  the  same  relative  direction,  and 
almost  in  the  same  plane ;  the  motions  of  the  satel- 
lites in  the  same  direction  as  those  of  the  planets; 
the  small  eccentricity  of  the  orbits  of  the  planets, 
upon  which  condition,  together  with  others  to  be 
noticed  in  a  subsequent  connection,  the  stability  of 
the  system  depends ;  and  the  position  of  the  source 
28 


326  TREATISE   ON    COMETS. 

of  light  and  heat  in  the  centre  of  the  system,  are 
all  satisfactory  by  this  hypothesis.  This  is  the 
nebular  hypothesis  of  Laplace,  and  although  it  was 
originally  advanced  as  simply  a  theory  of  the  de- 
velopment of  the  solar  system  from  one  primitive 
mass  of  nebulous  matter,  yet  the  facility  with  which 
it  explains  various  phenomena  connected  with  the 
structure  of  the  system  has  induced  many  to  regard 
it  as  strictly  true. 

Laplace  supposed  that  in  the  very  beginning  of 
the  creation  of  our  system,  the  sun  consisted  of  a 
diffused  and  extremely  attenuated  nebulous  matter, 
extending  over  a  vast  extent  of  space.  This  nebu- 
lous matter  may  have  been  so  exceedingly  attenuated 
that  its  existence  was  barely  a  fact.  As  a  confirma- 
tion of  this  view  of  the  case,  he  sought  an  analogy 
in  those  nebulae  which  are  now  seen  here  and  there 
throughout  the  entire  heavens,  so  feeble  and  diffused 
as  to  present  the  appearance  simply  of  an  ill-defined 
mass  of  vapor  when  seen  through  the  most  powerful 
telescopes.  He  imagined  that  in  the  vast  assem- 
blage of  irresolvable  nebulae  —  by  which  is  meant, 
those  which  the  most  powerful  telescopes  fail  to 
reduce  to  a  group  of  minute  stellar  points  of  light, 
or  individual  stars — there  were  examples  of  similar 
processes  of  development  now  going  on  in  various 
parts  of  the  universe.  In  the  first  place  we  see  the 
nebulous  matter  dispersed  in  patches  in  the  different 


NEBULAR   HYPOTHESIS.  327 

parts  of  the  sky,  while  in  some  of  these  it  is  sup- 
posed that  traces  are  exhibited  of  this  matter  feebly 
condensed  around  one  or  more  faint  nuclei.  In 
other  cases  these  nuclei  are  brighter  in  proportion 
to  the  surrounding  nebulosity,  and  further  on,  in 
the  apparent  progress  of  development,  there  are 
supposed  examples  of  the  condensation  of  the  atmo- 
sphere of  each  nucleus,  thus  exhibiting  a  collection 
or  aggregation  of  nebulous  stars,  formed  by  brilliant 
nuclei  very  near  each  other,  and  each  surrounded 
by  an  atmosphere.  Still  further  on  in  the  process 
of  development  the  nebulous  matter,  by  condensing 
uniformly,  is  supposed  to  form  nebulous  systems 
which  are  called  planetary;  and,  finally,  a  much 
greater  degree  of  condensation  transforms  all  these 
nebulous  systems  into  stars.  If  this  hypothesis  be 
true,  then  will  all  those  nebulse  which  are  now 
irresolvable,  even  in  the  most  powerful  telescopes, 
be  ultimately  transformed  into  stars,  while  it  may 
be  inferred,  from  a  similar  course  of  reasoning,  that 
the  anterior  or  primeval  condition  of  the  stars  which 
now  exist,  was  that  of  nebulous  matter  highly 
attenuated  and  distributed  throughout  space.  This 
is  the  most  general  view  of  the  nebular  hypothesis, 
and  granting  what  has  been  stated,  we  may  conclude 
that  at  this  very  moment  creation  is  going  on  in 
distant  parts  of  the  universe,  and  will  continue  for 
ever.  It  must  not  be  objected  that  there  is  anything 


328  TREATISE   ON   COMETS. 

in  this  view  of  the  creation  which  militates  against 
revealed  religion.  To  supply  the  primitive  matter, 
and  endow  it  with  such  properties  as  must  have 
been  required  to  effect  the  various  transformations 
from  inanimated  existence  to  animated  existence  of 
the  highest  order,  must  indicate  the  power  and 
presence  of  the  Almighty  Creator.  It  may  also  be 
objected  that  in  all  such  speculations  we  enter  the 
confines  of  the  unknown;  yet  the  anxious  search 
for  truth  which  characterizes  the  human  mind  must 
be  considered  a  sufficient  excuse  for  thus  wandering 
into  the  realms  of  the  invisible  and  uncertain. 

According  to  the  nebular  hypothesis  of  the  forma- 
tion of  new  worlds  from  original  and  primitive 
masses  of  matter,  the  comets  have  their  origin  in 
portions  of  the  nebulous  matter  occupying  positions 
intermediate  between  two  or  more  great  centres. 
These  masses  of  attenuated  matter  are  supposed  to 
be  held  in  a  state  of  equilibrium,  until,  finally,  the 
attraction  of  some  one  centre  predominates,  and 
the  uncondensed  filmy  mass  begins  to  descend 
slowly  toward  its  controlling  centre.  The  fact  that 
the  comets  come  into  our  system  in  all  possible 
directions,  and  in  orbits  which  conduct  them  far 
beyond  the  limits  of  our  system,  is  in  itself  conclu- 
sive evidence  to  show  that  they  wander  here  and 
there  among  the  various  systems  constituting  the 
universe.  This  would  naturally  be  supposed  to  be 


NEBULAR   HYPOTHESIS.  329 

the  case  if  they  were  formed  in  accordance  with  the 
nebular  hypothesis,  their  unconden,sed  condition 
being  due  to  the  fact  that  the  feeble  gravitation  of 
their  parts  toward  the  centre  of  the  mass  is  inade- 
quate to  contract  them  into  a  more  solid  form  ;  and 
also  that,  in  passing  near  the  suns  which  they  visit, 
they  are  necessarily  converted  by  the  intense  heat 
into  an  extremely  vaporous  state.  It  may  be  that 
in  the  course  of  ages  the  effects  of  the  cold  in  the 
remote  parts  of  their  orbits  would  solidify  them  to 
such  an  extent  that  in  future  they  might  retain,  to 
a  limited  extent,  a  solid  form ;  the  exterior  portions, 
however,  of  the  compact  nucleus,  in  their  successive 
revolutions,  going  through  alternate  changes  of 
solidification  and  evaporation.  We  may  remark  also 
that  the  idea  of  attributing  the  origin  of  comets  to 
the  nebular  hypothesis,  does  not  in* the  least  interfere 
with  or  modify  the  theory  of  the  formation  of  their 
tails  already  given. 

We  have  now  given  explanations  of  the  principal 
phenomena  presented  by  the  comets  directly,  which 
the  researches  of  astronomers  have  indicated,  and 
thus  far  our  task  is  accomplished.  There  is,  how- 
ever, one  other  consideration,  which  the  study  of 
the  motions  of  these  wonderful  bodies  has  presented, 
and  to  which  we  shall  next  advert;  namely,  the 
existence  of  a  medium  of  resistance  pervading  space, 
and  sensibly  opposing  the  motions  of  the  heavenly 
28* 


330  TREATISE   ON   COMETS. 

bodies,  at  least  those  which  are  known  as  cometary 
worlds.  We  cannot,  however,  conclude  the  con- 
sideration of  this  branch  of  our  subject  without  re- 
marking further,  that  the  phenomena  of  the  comets 
are  the  most  varied,  the  most  beautiful,  and,  at  the 
same  time,  fantastical,  which  the  heavens  present  to 
our  view.  The  peculiar  forms  of  their  orbits,  the 
suddenness  of  their  appearance,  and  the  develop- 
ment of  their  physical  changes  as  they  approach  the 
sun  and  again  recede  into  illimitable  space,  all  tend 
to  afford  a  sublime  conception  of  the  grandeur  of 
God's  creation.  Even  if  we  ever  feel  inclined  to 
doubt  in  regard  to  the  endless  extent  of  the  universe, 
the  circumstances  which  are  so  undeniably  proved 
to  be  connected  with  their  motions,  are  sufficient  to 
convince  us  of  the  reality  of  infinity.  Following, 
therefore,  these  mysterious  bodies  in  our  imagination 
in  their  endless  wanderings  through  space,  we  may 
perhaps  be  prepared  to  realize,  to  some  extent,  the 
beautiful  yet  true  conception  of  that  celebrated 
dream  of  Eichter : 

"God  called  up  from  dreams  a  man  into  the 
vestibule  of  heaven,  saying,  i  Come  thou  hither,  and 
see  the  glory  of  my  house.'  And  to  the  servants 
that  stood  around  his  throne  he  said,  '  Take  him, 
and  undress  him  from  his  robes  of  flesh  :  cleanse 
his  vision,  and  put  a  new  breath  into  his  nostrils ; 
only  touch  not  with  any  change  his  human  heart  — 


NEBULAR   HYPOTHESIS.  331 

the  heart  that  weeps  and  trembles.'  It  was  done: 
and,  with  a  mighty  angel  for  his  guide,  the  man 
stood  ready  for  his  infinite  voyage ;  and  from  the 
terraces  of  heaven,  without  sound  or  farewell,  at 
once  they  wheeled  away  into  endless  space.  Some- 
times with  solemn  flight  of  angel  wing  they  fled 
through  Zaarrahs  of  darkness,  through  wildernesses 
of  death,  that  divided  the  worlds  of  life  ;  sometimes 
they  swept  over  frontiers  that  were  quickening 
under  prophetic  motions  from  God.  Then,  from  a 
distance  that  is  counted  only  in  heaven,  light  dawned 
for  a  time  through  a  sleepy  film;  by  unutterable 
pace  the  light  swept  to  them,  they  by  unutterable 
pace  to  the  light.  In  a  moment  the  rushing  of 
planets  was  upon  them :  in  a  moment  the  blazing 
of  suns  was  around  them. 

"  Then  came  eternities  of  twilight,  that  revealed, 
but  were  not  revealed.  On  the  right  hand  and  on 
the  left  towered  mighty  constellations,  that  by  self- 
repetition  and  answers  from  afar,  that  by  counter- 
positions,  built  up  triumphal  gates,  whose  archi- 
traves, whose  arch-ways  —  horizontal,  upright  — 
rested,  rose  —  at  altitude  by  spans  —  that  seemed 
ghostly  from  infinitude.  Without  measure  were 
the  architraves,  past  number  were  the  arch-ways, 
beyond  memory  the  gates.  Within  were  stairs  that 
scaled  the  eternities  below ;  above  was  below  — 
below  was  above,  to  the  man  stripped  of  gravitating 


332  TREATISE   ON   COMETS. 

body:  depth  was  swallowed  up  in  height  insur- 
mountable, height  was  swallowed  up  in  depth  un- 
fathomable. Suddenly,  as  thus  they  rode  from  in- 
finite to  infinite,  suddenly,  as  thus  they  tilted  over 
abysmal  worlds,  a  mighty  cry  arose  —  that  systems 
more  mysterious,  that  worlds  more  billowy  —  other 
heights  and  other  depths — were  coming;  were  near- 
ing  ;  were  at  hand. 

"  Then  the  man  sighed,  and  stopped,  shuddered, 
and  wept.  His  overladened  heart  uttered  itself  in 
tears ;  and  he  said  —  'Angel,  I  will  go  no  further. 
For  the  spirit  of  man  acheth  with  this  infinity. 
Insufferable  is  the  glory  of  God.  Let  me  lie  down 
in  the  grave,  and  hide  me  from  the  persecution  of 
the  infinite;  for  end,  I  see,  there  is  none.'  And 
from  all  the  listening  stars  that  shone  around  issued 
a  choral  voice,  '  The  man  speaks  truly :  end  there  is 
none  that  ever  yet  we  heard  of.'  'End  is  there 
none?'  the  angel  solemnly  demanded:  'Is  there 
indeed  no  end  ?  —  and  is  this  the  sorrow  that  kills 
you  ? '  But  no  voice  answered  that  he  might  answer 
himself.  Then  the  angel  threw  up  his  glorious 
hands  toward  the  heaven  of  heavens,  saying,  'End 
is  there  none  to  the  universe  of  God.  Lo  !  also 
there  is  no  beginning.'  " 


RESISTING  MEDIUM  IN  SPACE.        333 


CHAPTER    V". 

ON  THE  EXISTENCE  OF  A  RESISTING  MEDIUM    IN    SPACE CHARACTER 

OF  THE  RESISTING  MEDIUM ITS    EFFECT    ON  THE  MOTIONS  OF  THE 

HEAVENLY     BODIES  —  THE     STABILITY    OF     THE     SOLAR     SYSTEM  — 
INFLUENCE    OF    THE    COMETS GENERAL    REMARKS. 

THE  question  of  a  plenum  and  a  vacuum  has  en- 
gaged the  attention  of  philosophers  from  the  earliest 
ages  of  the  world.  In  their  speculations  respecting 
the  constitution  of  the  universe,  it  often  became  a 
matter  of  doubt  whether  the  celestial  and  terrestrial 
spaces  are  absolutely  full,  or  whether  there  are,  be- 
tween and  among  the  material  parts  of  the  universe, 
empty  spaces  free  from  all  matter.  The  general 
opinion,  however,  seems  to  have  been  that  the 
regions  around  us,  beyond  our  atmosphere,  and  to 
an  indefinite  extent,  are  supplied  with  a  rare  invisi- 
ble medium,  of  unknown  composition  and  character, 
in  which  all  bodies  are  compelled  to  move.  In  the 
astronomy  of  the  Brahmins  of  India,  which  dates 
its  origin  long  before  the  commencement  of  the 
Christian  era,  the  existence  of  such  a  medium  is 
assumed,  though  the  stars  were  by  them  supposed 
to  move  in  it  like  fishes  in  water.  They  regarded 
it  as  a  celestial  element,  pure  and  impalpable,  and 


334  TREATISE   ON    COMETS. 

offering  no  resistance  to  the  motions  of  the  heavenly 
bodies.  Indeed,  almost  every  ancient  astronomer 
held  similar  opinions.  Kepler,  notwithstanding  his 
profound  knowledge  of  astronomy,  in  seeking  the 
origin  of  comets,  supposed  them  to  he  native  in- 
habitants of  this  fluid  medium,  as  the  fishes  are  of 
the  waters  of  the  earth ;  and  that  they  were  thus 
created  to  inhabit  the  immense  spaces  of  the  uni- 
verse. The  sombre  and  bloody  appearance  which 
the  sun  sometimes  exhibits,  he  attributed  to  a 
coagulation  of  the  ethereal  fluid,  and  the  cessation 
of  these  appearances  he  supposed  to  be  due  to  the 
collection  of  the  denser  portions  of  the  ether  in  the 
form  of  comets. 

The  theory  of  an  ethereal  medium  throughout  all 
space  remained  thus  until  the  development  of  a 
system  of  the  world  by  Descartes.  He  supposed 
that  matter  possessed  only  the  properties  of  ex- 
tension, impenetrability,  and  inertia;  that  it  filled 
all  space ;  and  that  it  was,  in  its  parts,  both  great 
and  small,  endued  with  motion  in  an  infinity  of 
directions.  He  imagined  that  the  matter  would 
thus  be  formed  into  innumerable  vortices,  differing 
not  only  in  extent,  but  also  in  velocity  and  density ; 
the  more  attenuated  parts  constituting  the  real 
vortex,  in  which  the  denser  bodies  float,  and  by 
which  they  are  pressed,  though  not  equally,  on  all 
sides.  He  therefore  regarded  the  universe  as  con- 


RESISTING  MEDIUM  IN  SPACE.        835 

sisting  of  innumerable  vortices,  which  limit  and 
circumscribe  one  another,  and  supposed  that  the 
earth  and  planets  are  bodies  carried  round  in  the 
great  vortex  of  the  solar  system.  He  supposed  also, 
that,  by  the  pressure  of  the  attenuated  matter  which 
he  had  assumed  to  fill  all  space,  and  which  circu- 
lates with  great  rapidity  and  great  centrifugal  force, 
the  denser  bodies,  which  have  less  rapidity  and  less 
centrifugal  force,  are  forced  toward  the  sun,  the 
centre  of  the  vortex.  In  like  manner,  each  planet 
was  itself  regarded  as  the  centre  of  a  small  vortex, 
by  the  operation  of  which  all  the  phenomena  of 
gravity  would  be  produced.  This  is  the  Cartesian 
system  of  the  world,  a  theory  which  was  very  gene- 
rally received  for  nearly  half  a  century  from  the 
date  of  its  promulgation  ;  and  it  is  evident  that  the 
existence  of  an  ethereal  fluid  pervading  all  space  is 
here  adopted  as  the  basis  of  the  theory.  The  glaring 
defects,  however,  of  the  doctrine  of  Descartes,  were 
conclusively  established  by  the  subsequent  disco- 
veries of  Newton  and  Kepler.  It  was  shown  by  the 
former,  that  the  planetary  and  cometary  bodies  re- 
volve around  the  sun  by  virtue  alone  of  the  force 
of  gravitation,  and  the  original  force  of  projection 
imparted  to  them  at  the  time  of  their  formation. 
That  this  theory  was  correct,  the  complete  verifica- 
tion of  Kepler's  laws  exhibited  the  most  incontesta- 
ble evidence,  while  the  subsequent  development  of 


336  TREATISE   ON   COMETS. 

the  great  structure  which  modern  astronomy  forms, 
has  completely  verified  and  generalized  it  in  every 
particular. 

The  Newtonian  theory  of  gravitation  was  con- 
ceived by  some  to  require  that  the  motions  of  the 
heavenly  bodies  should  be  performed,  according  to 
mechanical  laws,  in  space  which  was  absolutely  a 
vacuum  ;  and  hence,  that  the  doctrine  of  an  ethereal 
medium  of  any  conceivable  density  was  inadmissi- 
ble. This,  however,  is  by  no  means  a  necessary 
consequence  of  the  law  of  universal  gravitation,  and 
was  not  so  considered  by  Newton  himself.  He  be- 
lieved firmly  in  the  existence  of  a  subtile  ethereal 
fluid  pervading  all  space,  and  even  the  internal  parts 
of  all  material  bodies.  He  supposed  that  possibly 
it  was  this  medium  which  furnished  the  means  for 
the  operation  of  the  gravitating  force.  The  ethe- 
real fluid  was  conceived  of  as  a  sort  of  spirit,  by  the 
force  and  action  of  which  the  particles  of  bodies 
mutually  attract  each  other  at  near  distances,  and 
cohere  if  contiguous.  He  imagined  that  possibly 
this  medium  is  much  rarer  within  the  dense  bodies 
of  the  sun,  stars,  planets,  and  comets,  than  in  the 
empty  spaces  between  them;  and  that  in  passing 
from  them  to  great  distances,  it  became  gradually 
denser  and  denser,  thus,  it  may  be,  causing  the 
gravity  of  these  bodies  toward  each  other,  and  of 
their  denser  parts  toward  a  centre ;  every  body  tend- 


RESISTING  MEDIUM  IN  SPACE.        337 

ing*  continually  to  move  from  the  denser  portions 
of  the  medium  toward  the  rarer.  These  hypotheses 
in  regard  to  the  existence,  nature,  and  effect  of  the 
ethereal  fluid,  were  not,  however/advanced  by  Few- 
ton  as  any  part  of  his  theory  of  universal  gravita- 
tion, but  simply  as  a  possible  method  of  explaining 
the  operation  of  the  gravitating  force.  This  was 
all  that  it  was  necessary  for  him  to  maintain,  since 
his  theory  of  the  system  of  the  world  was  founded 
exclusively  on  observations,  and  had  nothing  to  do 
with  abstract  possibilities  and  metaphysical  neces- 
sities. 

The  motions  of  the  planets  computed  strictly  in 
accordance  with  the  law  of  gravitation,  and  under 
the  supposition  that  they  move  in  an  absolute 
vacuum,  or,  at  least,  in  a  space  void  of  any  resisting 
fluid,  were  found  to  accord  so  closely  with  observa- 
tion, that  it  was  for  some  time  doubted  whether  any 
such  medium  as  Newton  and  others  had  suspected, 
really  existed.  The  phenomena  of  the  planetary 
motions  were  so  accurately  predicted  by  the  New- 
tonian theory  of  gravitation,  that  it  became  evident 
either  that  there  was  no  such  fluid,  or  that  it  was  so 
extremely  attenuated  or  rarefied,  that  no  phenome- 
non which  had  hitherto  been  observed  was  capable 
of  exhibiting  its  effects.  In  order,  however,  to  test 
this  question  thoroughly,  the  Academy  of  Sciences, 
of  Paris,  in  1762,  offered  a  prize  for  the  solution  of 
29  w 


338  TREATISE   ON   COMETS. 

the  following  definite  problem :  "Do  the  planets  re- 
volve in  a  medium  of  which  the  resistance  produces  a 
sensible  effect  upon  their  movements  ?"  The  prize  was 
awarded  to  Bossut,  who  found,  by  calculation,  that 
the  effect  of  any  such  resistance,  offered  to  the 
planets,  would  be  to  diminish  the  greater  axis  of 
their  orbits,  and,  consequently,  to  shorten  their 
periods  of  revolution.  The  moon  was  known  to 
revolve  in  periods  which  were  becoming  shorter  and 
shorter,  and  he  concluded  that  the  observed  accelera- 
tion was  due  to  the  resistance  of  the  ether.  He 
applied  the  same  reasoning  in  the  case  of  the  planets, 
and  although  no  such  acceleration  could  be  detected, 
yet  he  conjectured  that  in  the  course  of  time  similar 
effects  would  be  exhibited.  It  should  be  remarked, 
however,  that  up  to  the  present  time  no  acceleration 
of  the  planets,  due  to  action  of  a  resisting  medium, 
has  been  detected;  and  the  acceleration  of  the 
moon's  mean  motion  is  explained  thoroughly  and 
satisfactorily  by  the  changes  which  the  perturba- 
tions by  the  sun  and  planets  are  producing  in  the 
eccentricity  of  the  earth's  orbit — thus  showing  con- 
clusively, that  so  far  as  the  motions  of  the  planets 
and  their  satellites  are  concerned,  we  have  no  evi- 
dence of  the  existence  of  a  resisting  ethereal  fluid 
in  space. 

We  have  already  stated  the  fact  that  light  is  cer- 
tainly known  to  be  produced  by  successive  undula- 


RESISTING  MEDIUM  IN  SPACE.         339 

tions  or  pulsations  in  an  extremely  attenuated  and 
elastic  ethereal  fluid,  which  pervades  all  space ;  and 
that  heat  and  electricity  may  with  great  probability 
be  attributed  to  the  operation  of  the  same  cause, 
only  under  different  circumstances.  Such  being  the 
case,  it  seems  evident  that  this  ethereal  fluid  will 
oppose  itself  to  the  motions  of  the  heavenly  bodies 
in  their  orbits,  and  that  its  effect  on  the  motions  of 
the  comets  would  be  first  exhibited,  since  the  ex- 
tremely vaporous  matter  of  which  they  are  com- 
posed would  be  most  readily  operated  upon  by  the 
resisting  medium.  If,  therefore,  it  should  be  found 
that  the  comets  which  return  regularly  and  at  short 
intervals,  actually  exhibit  the  accelerated  motion 
which  would  result  from  the  operation  of  such  a 
medium  of  resistance,  it  may  be  regarded  as  con- 
clusively established,  that  the  undulatory  theory  of 
light  is  a  physical  fact,  and  that  there  exists  through- 
out this  illimitable  universe  a  subtile  ethereal  fluid. 
The  only  objection  which  can  be  urged  against  this 
assumption  is  that  the  resisting  medium  must  neces- 
sarily be  composed  of  ponderable  matter,  while  the 
ethereal  fluid,  which  serves  for  the  transmission  of 
light,  is  supposed  to  be  strictly  imponderable.  This 
objection  has  indeed  been  urged  in  opposition  to 
the  theory  of  a  resisting  medium,  but  it  should  be 
remembered  that  the  idea  of  imponderability  is 
essentially  relative.  The  comets  themselves,  to  a 


340  TREATISE   ON    COMETS. 

limited  extent,  are  imponderable,  and  when  it  is 
considered  that  the  tails  of  these  bodies  are  so  ex- 
tremely attenuated  that  they  approach  almost  to 
the  nature  of  the  invisible  ether,  the  identity  of  the 
resisting  medium  which  opposes  their  motions,  and 
the  ethereal  fluid  which  transmits  light,  is  by  no 
means  doubtful.  The  phenomena  of  double  refrac- 
tion and  polarization  have  demonstrated  with  un- 
erring certainty  that  the  undulatory  theory  of  light, 
which  presupposes  the  existence  of  a  subtile  ethe- 
real fluid  throughout  space,  is  the  only  one  which 
can  be  recognized  in  the  present  state  of  optical 
science ;  while  other  considerations,  which  have 
already  been  stated,  tend  to  prove  that  there  exists 
some  common  and  yet  inscrutable  bond,  which  con- 
nects these  powerful  agents  of  nature.  There  can 
be  no  doubt,  therefore,  even  if  there  were  not 
additional  phenomena,  and  of  an  entirely  different 
character,  tending  to  the  same  final  consequence, 
but  that  the  ethereal  fluid  exists  in  every  part  of  the 
universe  in  which  creation  is  already  begun  or 
ended,  and  that  this  subtile  fluid  must  necessarily 
oppose  itself  to  the  free  motion  of  all  material  bodies. 
There  is  nothing  in  this  view  of  the  system  of  the 
world  which  can  be  objected  to  on  account  of  any 
assumed  insufficiency  of  the  data  from  which  such 
conclusions  have  been  derived,  or,  as  being  in  con- 
travention with  the  teachings  of  revelation. 


RESISTING  MEDIUM  IN  SPACE.         341 

The  existence  of  a  resisting  medium  in  space  may 
thus  be  supposed  to  be  established  on  a  purely 
theoretical  basis,  and  it  remains  now  to  see  whether 
there  are  any  phenomena  connected  with  the  motions 
of  any  of  the  heavenly  bodies,  by  means  of  which, 
even  if  no  such  ideas  had  been  previously  advanced, 
the  same  conclusion  would  have  been  derived.  The 
planets  do  not,  during  the  period  in  which  they  have 
been  accurately  observed,  afford  any  indication  of 
an  opposing  force,  such  as  would  result  from  a 
resisting  medium.  Their  great  weight,  compared 
with  the  extent  of  their  volume,  would  necessarily 
render  them  less  liable  to  be  sensibly  affected  by 
such  a  force  of  resistance.  The  case  of  the  comets, 
however,  is  far  different.  These  bodies  are  them- 
selves composed,  for  the  most  part,  of  matter  so 
attenuated,  while  the  extent  of  their  volume  is  often 
enormous,  that  the  greatest  resistance  would  be 
opposed  to  their  motions ;  and,  consequently,  it  is 
to  these  bodies  that  we  are  to  look  for  an  exemplifi- 
cation of  the  effects  of  a  resisting  medium.  It  has 
already  been  noticed  in  the  case  of  Halley's  comet 
that  the  effect  of  a  resisting  medium  on  its  motions, 
from  1682  to  1759,  was  computed  by  Clairaut,  and 
found  to  be  so  small  as  "to  be  almost,  if  not  entirely 
insensible.  Such  being  the  case,  the  effect  of  a 
resistance  from  the  ethereal  fluid  was  not  considered 
in  the  computations  which  were  made  for  its  return 
29* 


842  TREATISE   ON    COMETS. 

in  1835.  It  has  also  been  noticed  in  the  case  ot 
Encke's  comet,  and  of  Biela's  comet,  that  their  suc- 
cessive returns  to  their  perihelia  take  place  at  in- 
tervals which  are  regularly  becoming  shorter  and 
shorter.  This  diminution  of  the  period  of  these 
comets  has  been  attributed  to  the  influence  of  a 
resisting  medium,  and  the  amount  of  the  diminu- 
tion of  the  period  corresponds  precisely  with  what 
might  be  expected  to  result  from  such  a  cause.  In 
the  case  of  Encke's  comet  the  diminution  of  the 
period  has  been  determined  with  the  very  greatest 
precision,  the  short  period  of  its  revolution  afford- 
ing great  facilities  for  settling  the  question  as  to 
the  existence  of  a  medium  of  resistance.  Encke  has 
computed  the  planetary  perturbations  for  the  entire 
period  which  has  elapsed  since  the  comet  first  be- 
came known  to  astronomers,  and  finds  that,  after 
making  due  allowance  for  these  disturbances,  its 
period  is  shorter  at  each  successive  return.  The 
amount  of  the  diminution  of  its  periodic  time  is 
very  nearly  eleven-hundredths  of  a  day,  or  two 
hours,  thirty-eight  minutes,  and  twenty-four  seconds, 
at  each  return.  The  same  result  has  been  obtained 
in  the  case  of  Biela's  comet,  but  since  the  perturba- 
tions have  not  yet  been  computed  for  its  successive 
returns  with  extreme  accuracy,  and,  moreover,  on 
account  of  the  anomalous  phenomenon  of  its  having 
separated  into  two  separate  and  distinct  comets,  the 


RESISTING  MEDIUM  IN  SPACE.        343 

exact  amount  of  the  acceleration  lias  not  been  ascer- 
tained. There  can  be  no  doubt,  however,  but  that 
its  period  is  becoming  shorter  at  each  successive 
return  to  its  perihelion,  and  that  the  other  comets  of 
short  period  will  eventually,  after  a  sufficient  num- 
ber of  revolutions,  be  found  to  exhibit  precisely  the 
same  phenomenon.  It  may,  therefore,  be  considered 
as  well  established  that  the  motions  of  the  comets 
indicate,  beyond  all  doubt,  the  existence  of  a  resist- 
ing medium  within  the  limits  included  by  the  solar 
system,  while  the  identity  of  this  with  th^  ethereal 
fluid  which  transmits  light,  being  admitted,  shows 
that  the  same  medium  of  resistance  is  universal  in 
the  material  universe. 

The  effect  of  a  resisting  medium  on  the  motions 
of  the  bodies  which  revolve  around  the  sun  will  be 
to  continually  accelerate  their  mean  motion,  and, 
consequently,  shorten  their  period  of  revolution. 
The  eccentricity  of  their  orbits  will  gradually 
dimmish,  as  the  greater  axis  decreases,  and,  finally, 
they  will  one  by  one  be  plunged  into  the  sun,  after 
having  described  an  indefinite  number  of  circum- 
volutions of  a  kind  of  spiral  curve,  wThich  gradually 
at  first,  but  afterward  rapidly  brings  them  to  the 
central  body.  The  comets  being  most  affected  by 
the  resistance  will  necessarily  be  the  first  to  fall  to 
the  sun,  and  then  in  rapid  succession  the  various 
planets  belonging  to  our  system ;  until  finally  all 


344  TREATISE   ON    COMETS. 

traces  of  the  former  existence  of  a  beautiful  and 
yet  complicated  system  of  comets  and  planets  will 
be  completely  obliterated.  This  will  be  the  neces- 
sary result  of  the  resistance  of  an  ethereal  fluid  in 
space,  though  it  shall  be  accomplished  only  after 
the  lapse  of  almost  an  indefinite  number  of  ages. 
It  may,  therefore,  011  account  of  our  preconceived 
idea  of  infinite  duration,  seem  very  improbable,  if 
there  be  not  other  forces  which  can  overcome  the 
eifect  of  a  resisting  medium,  that  it  can  really  exist, 
notwithstanding  the  facts  already  presented.  The 
conception  of  universal  gravitation  is  so  simple  and 
yet  so  beautiful,  that  when  we  come  to  consider  its 
legitimate  results,  in  connection  with  the  existence 
of  a  great  central  sun,  around  which  the  systems 
and  compound  systems  of  the  universe  revolve,  the 
mind  hesitates  to  assume  the  existence  of  any  force 
either  of  repulsion  or  of  resistance,  which  can  limit 
the  period  of  these  revolutions  and  counter-revolu- 
tions. For  example,  supposing  it  possible  to  view 
our  own  solar  system  from  some  point  in  space 
beyond  its  limits,  and  that  it  is  possible  to  witness 
both  the  revolutions  of  the  planets  and  comets 
around  the  sun,  and  the  revolution  of  the  entire 
system  around  its  distant  centre,  let  us  contemplate 
in  imagination  the  scene  which  would  be  presented 
to  our  view.  In  the  centre  of  the  system  would  be 
seen  the  great  central  orb,  or  sun,  and  around  him 


RESISTING  MEDIUM  IN  SPACE.         345 

the  planets,  some  with  and  some  without  satellites, 
each  performing  its  appointed  revolution.  Here 
and  there  among  these  would  be  seen  chaotic  worlds, 
with  streams  of  light  flowing  from  them,  some 
moving  rapidly  toward  the  central  body  with  a 
constantly  accelerated  velocity,  and  others  moving 
from  it  with  a  velocity  decreasing  just  as  in  the 
other  case  it  increased,  while  with  all  this  gorgeous 
equipage  of  glittering  worlds,  the  great  sun  itself 
and  system  would  be  seen  to  move  slowly  but  uni- 
formly around  its  centre.  This  also  might  be  con- 
ceived of  as  revolving  around  a  second  great  centre, 
and  this  again  around  a  third,  and  so  on  until, 
finally,  the  mind  rests  on  the  contemplation  of  a 
great  centre  of  centres  of  revolution,  around  which 
the  entire  universe  is  brought  to  an  equipoise.  This 
conception  of  the  structure  of  the  universe  may 
perhaps  be  regarded  as  fanciful,  and  doubtless,  to  a 
greater  or  less  extent,  it  is  so ;  yet  there  is  in  it  that 
which  causes  the  reason  at  first  to  revolt  against  the 
assumption  of  the  existence  of  any  forces  in  nature 
which  may  ultimately  destroy  the  present  state  of 
things,  as  contrary  to  the  evident  design  of  2n 
intelligent  and  omnipotent  Creator.  To  this  may 
be  traced  the  great  opposition  which  the  theory  of 
the  existence  of  a  resisting  medium  in  space  has  re- 
ceived, and  which  has  led  to  various  other  hypotheses 


346  TREATISE   ON    COMETS. 

to  explain  the  phenomena  of  the  diminution  of  the 
periodic  times  of  the  comets. 

Without  attempting  to  consider  the  various  theo- 
ries which  have  heen  devised  in  order  to  avoid  the 
assumption,  or  rather  the  confirmation,  of  the  ex- 
istence of  an  ethereal  fluid  pervading  all  space, 
which  opposes  itself  to  the  motions  of  the  heavenly 
bodies,  let  it  suffice  simply  to  state  that  all  these  are 
so  incomplete  and  unsatisfactory,  that,  however 
much  we  may  feel  inclined  to  do  otherwise,  we  are 
compelled  finally  to  resort  to  the  hypothesis  of  a 
resisting  medium  in  order  to  explain  the  phenonienq, 
observed.  This  we  are  compelled  to  do  in  order  to 
satisfy  the  results  of  observation,  while,  for  reasons 
which  have  already  been  given,  we  must  admit  the 
existence  of  an  ethereal  fluid,  whether  sufficiently 
dense  or  not  to  oppose  sensibly  the  motions  of  the 
heavenly  bodies.  It  may  be  perceived,  therefore, 
that  the  theory  of  a  resisting  medium  presents  the 
very  greatest  claims  to  our  consideration,  as  the 
only  one  which  can  explain  simultaneously  several 
of  the  most  important  facts  connected  with  the  con- 
stitution of  the  material  universe;  and,  consequently, 
before  proceeding  to  investigate  more  fully  its  ulti- 
mate effect  on  our  system,  it  may  not  be  improper 
to  notice  briefly  the  provisions  which  the  operation 
of  the  law  of  gravitation  has  afforded  for  securing 
the  permanence  of  the  system. 


RESISTING  MEDIUM  IN  SPACE.        347 

From  what  has  previously  been  stated  in  regard 
to  the  mutual  attractions  of  the  heavenly  bodies,  it 
may  be  understood  that,  since  all  the  planets  are 
small  compared  with  the  sun,  the  derangement 
which  they  may  produce  on  any  one  of  their  number 
will  be  very  small  in  the  course  of  a  few  revolutions. 
But  since  the  same  forces  are  constantly  in  opera- 
tion, and  must  continue  so,  it  may  be  easily  con- 
ceived that  in  the  course  of  ages  the  derangement 
of  the  planetary  motions  may  accumulate,  the  orbits 
may  change  their  form,  and  their  mutual  distances 
may  be  greatly  increased  or  diminished.  These 
changes  might  be  supposed  to  continue  without 
limit,  and  end  finally,  though  perhaps  at  a  date 
almost  infinitely  remote,  in  the  total  subversion  and 
ruin  of  the  system.  Moreover,  the  careful  obser- 
vation of  the  planets  for  a  long  period  of  time  has 
shown  that  changes  are  actually  taking  place  in 
their  relative  motions,  of  such  a  nature  as  just  re- 
ferred to,  which  have  been  going  on  progressively 
since  the  very  dawn  of  science.  The  eccentricity 
of  the  orbit  of  the  earth  has  been  decreasing  for 
thousands  of  years,  and,  consequently,  the  mean 
motion  of  the  moon  has  become  more  and  more 
accelerated.  The  moon,  therefore,  perpetually  ap- 
proaches nearer  and  nearer  the  earth  at  each  revo- 
lution, and  should  this  change  Continue  forever,  it 
would  eventually  fall  to  our  globe,  an  event  which 


348  TREATISE   ON    COMETS. 

would  indeed  be  a  dreadful  calamity.  The  question 
may  thus  arise,  whether  these  and  similar  changes 
will  continue  indefinitely,  or  whether,  in  the  nature 
of  the  planetary  orbits,  there  exist  conditions  which 
fix  the  limit  of  all  these  changes,  and  thus  preserve 
the  stability  of  the  system. 

The  problem  which  is  thus  presented  is  certainly 
worthy  of  the  highest  exertions  of  the  human  mind, 
while  its  solution  is  a  task  of  no  ordinary  difficulty. 
The  problem  is  indeed  one  of  such  proportions  that 
no  attempt  was  made  to  demonstrate  either  the 
stability  or  instability  of  the  solar  system,  until 
near  the  close  of  the  eighteenth  century.  It  was 
then  shown  by  Lagrange  and  Laplace,  that  the 
arrangements  of  the  system  are  stable,  that  in  the 
course  of  time  counter-changes  will  take  place,  so 
that  in  the  end  the  planetary  orbits  and  motions 
remain  unchanged.  The  variations  of  the  elements 
of  their  orbits,  caused  by  their  mutual  attractions 
in  accordance  with  the  law  of  universal  gravitation, 
are  periodical,  and  not  indefinitely  progressive.  The 
periods,  however,  in  which  these  changes  and 
counter-changes  are  effected,  are  indeed  enormous ; 
not  less  than  thousands,  and,  in  some  instances, 
millions  of  years  ;  and  hence  it  is  that  some  of  these 
apparent  derangements  have  been  progressing  in 
the  same  direction  since  the  very  beginning  of  the 
history  of  the  world.  The  disturbances  which  will 


STABILITY  OF  THE  SOLAR  SYSTEM.      349 

ever  exist  will  not  be  sufficient  to  modify  or  alter 
the  adaptations  of  the  system. 

It  was  demonstrated  by  Laplace,  that  whatever 
be  the  masses  of  the  planets,  in  consequence  of  the 
fact  that  they  all  move  in  the  same  direction  in 
orbits  of  small  eccentricity,  and  slightly  inclined  to 
each  other  —  their  secular  inequalities  or  changes 
are  periodical,  and  included  within  narrow  limits; 
so  that  the  planetary  system  will  only  oscillate  about 
a  mean  state,  and  will  never  deviate  from  it,  except 
by  a  small  quantity.  The  orbits  of  the  planets  have 
been,  and  will  always  be,  nearly  circular.  The 
ecliptic  will  never  coincide  with  the  equator,  and 
the  entire  extent  of  the  variation  in  its  inclination 
cannot  exceed  three  degrees.  Thus  will  the  planets, 
so  far  as  gravity  alone  is  concerned,  continue  to 
revolve  forever  in  orbits,  whose  planes  will  rock 
slowly  up  and  down,  gradually  contracting  and  ex- 
panding, but  yet  compelled  to  oscillate  about  a  mean 
orbit  whose  form  and  position  is  fixed. 

Such  is  the  result  of  computations  which  are  of 
the  most  positive  character;  and  it  is  beyond  all 
doubt  that  if  the  force  of  gravity  alone  operates 
on  the  bodies  of  the  system,  it  will  ever  continue, 
so  far  as  the  effect  of  the  planets  may  be  considered, 
to  preserve  the  stability  of  the  system.  By  it  these 
changes  are  produced,  and  by  it  the  limits  of  the 
changes  themselves  are  fixed.  These  limits  can 
30 


350  TREATISE   ON    COMETS. 

never  be  passed,  and  after  the  lapse  of  perhaps 
millions  of  ages,  the  entire  system  will  have  resumed 
its  primitive  condition.  But  if  now  we  introduce 
into  the  system  an  opposing  force,  such  as  would 
result  from  a  resisting  medium,  the  beautiful  ar- 
rangement of  the  configurations  of  the  planets 
which,  by  the  operation  of  the  gravitating  force 
preserves  and  perpetuates  the  conditions  of  stability, 
will  no  longer  exist.  Unless,  therefore,  we  can  con- 
ceive of  other  forces  in  nature  which  can  compen- 
sate for  the  effect  of  a  resisting  medium,  we  must 
regard  it  as  inevitably  decreed  that  all  must  end. 
This  would  be  a  necessary  and  unavoidable  result, 
and  its  final  consummation  will  depend  simply  on  the 
character  of  the  medium  which  exists.  Thus,  it 
has  been  found  that  Encke's  comet  would  lose  one- 
half  of  its  present  mean  velocity  in  less  than  twenty- 
three  thousand  years.  In  a  similar  manner,  it  has 
been  found,  or  rather  conjectured,  that  if  Jupiter 
were  to  lose  a  millionth  part  of  his  velocity  in  a 
million  of  }^ears  —  which  is  regarded  as  far  more 
than  may  be  considered  in  any  way  probable  —  he 
would  require  seventy  millions  of  years  to  lose  one- 
thousandth  of  his  present  velocity,  and  a  period 
seven  hundred  times  as  long  to  reduce  it  to  one-half. 
These  immense  periods  are  almost  sufficient  to  over- 
whelm the  imagination  —  nevertheless  it  is  certain 
that  even  if  such  changes  are  to  be  ultimately 


STABILITY  OF  THE  SOLAR  SYSTEM.      351 

effected,  the  time  which  must  elapse  will  be  much 
greater.  It  may  indeed  be  millions  of  millions  of 
years  before  the  earth's  retardation  will  sensibly 
change  the  length  of  our  year  and  the  course  of 
our  seasons,  yet,  if  there  are  no  forces  to  counter- 
balance, even  this  exemplification  of  the  smallness 
of  the  resistance  does  not  in  the  least  indicate  its 
uncertainty  or  that  of  the  fate  which  thus  inevitably 
awaits  our  planet  and  its  inhabitants,  should  they 
be  permitted  to  exist  for  so  long  a  time.  The  same 
effect  will  be  produced  throughout  the  system,  and 
unless  counter-changes  shall  be  produced,  it  is  indeed 
an  appalling  fact  that  all  must  end. 

Such  considerations  as  these  have  induced  Whe- 
well  to  indulge  in  the  following  reflections :  "  The 
vast  periods  which  are  brought  under  our  conside- 
ration in  tracing  the  effects  of  the  resisting  medium, 
harmonize  with  all  that  we  learn  of  the  constitution 
of  the  universe  from  other  sources.  Millions  and 
millions  of  years  are  expressions  that  at  first  sight 
appear  fitted  only  to  overwhelm  and  confound  all 
our  powers  of  thought;  and  such  numbers  are  no 
doubt  beyond  the  limits  of  anything  which  we  dis- 
tinctly conceive.  But  our  powers  of  conception  are 
suited  rather  to  the  wants  and  uses  of  common  life, 
than  to  a  complete  survey  of  the  universe.  It  is  in 
no  way  unlikely  that  the  whole  duration  of  the 
solar  system  should  be  a  period  immeasurably  great 


352  TREATISE   ON   COMETS. 

in  our  eyes,  though  demon str ably  finite.  Such 
enormous  numbers  have  been  brought  under  our 
notice  by  all  the  advances  we  have  made  in  our 
knowledge  of  nature.  The  smallness  of  the  objects 
detected  by  the  microscope  and  of  their  parts ;  — 
the  multitude  of  the  stars  which  the  best  telescopes 
of  modern  times  have  discovered  in  the  sky ;  —  the 
duration  assigned  to  the  globe  of  the  earth  by 
geological  investigation  ;  —  all  these  results  require 
for  their  probable  expression,  numbers  which,  so 
far  as  we  see,  are  on  the  same  gigantic  scale  as  the 
number  of  years  in  which  the  solar  system  will 
become  entirely  deranged.  Such  calculations  de- 
pend in  some  degree  on  our  relation  to  the  vast 
aggregate  of  the  works  of  our  Creator;  and  no 
person  who  is  accustomed  to  meditate  on  these 
subjects  will  be  surprised  that  the  numbers  which 
such  an  occasion  requires  should  oppress  our  com- 
prehension. No  one  who  has  dwelt  on  the  thought 
of  a  universal  Creator  and  Preserver,  will  be  sur- 
prised to  find  the  conviction  forced  upon  the  mind 
by  every  new  train  of  speculation,  that  viewed  in 
reference  to  Him,  our  space  is  a  point,  our  time  a 
moment,  our  millions  a  handful,  our  permanence  a 
quick  decay. 

"  Our  knowledge  of  the  vast  periods,  both  geo- 
logical and  astronomical,  of  which  we  have  spoken, 
is  most  slight.  It  is  in  fact  little  more  than  that 


STABILITY  OF  THE  SOLAR  SYSTEM.      353 

such  periods  exist;  that  the  surface  of  the  earth 
has,  at  wide  intervals  of  time,  undergone  great 
changes  in  the  disposition  of  land  and  water,  and 
in  the  forms  of  animal  life ;  and  that  the  motions 
of  the  heavenly  bodies  round  the  sun  are  effected, 
though  with  inconceivable  slowness,  by  a  force 
which  must  end  by  deranging  them  altogether.  It 
would,  therefore,  be  rash  to  endeavor  to  establish 
any  analogy  between  the  periods  thus  disclosed; 
but  we  may  observe  that  they  agree  in  this,  that 
they  reduce  all  things  to  the  general  rule  of  finite 
duration.  As  all  the  geological  states  of  which  we 
find  evidence  in  the  present  state  of  the  earth,  have 
had  their  termination,  so  also  the  astronomical  con- 
ditions under  which  the  revolutions  of  the  earth 
itself  proceed,  involve  the  necessity  of  a  future 
cessation  of  these  revolutions. 

"  The  contemplative  person  may  well  be  struck 
by  this  universal  law  of  the  creation.  We  are  in 
the  habit  sometimes  of  contrasting  the  transient 
destiny  of  man  with  the  permanence  of  the  forests, 
the  mountains,  the  ocean  —  with  the  unwearied  cir- 
cuit of  the  sun.  But  this  contrast  is  a  delusion  of 
our  own  imagination ;  the  difference  is  after  all  but 
one  of  degree.  The  forest  tree  endures  for  its 
centuries,  and  then  decays ;  the  mountains  crumble 
and  change,  and  perhaps  subside  in  some  convulsion 
of  nature ;  the  sea  retires,  and  the  shore  ceases  to 
30*  x 


354  TREATISE   ON   COMETS. 

resound  with  the  '  everlasting  '  voice  of  the  ocean  ; 
such  reflections  have  already  crowded  upon  the 
mind  of  the  geologist;  and  it  now  appears  that  the 
courses  of  the  heavens  themselves  are  not  exempt 
from  the  universal  law  of  decay ;  that  not  only  the 
rocks  and  the  mountains,  but  the  sun  and  moon, 
have  the  sentence,  'to  end,'  stamped  upon  their 
foreheads.  They  enjoy  no  privilege  beyond  man 
except  a  longer  respite.  The  ephemeron  perishes 
in  an  hour ;  man  endures  for  his  three-score  years 
and  ten  ;  an  empire,  a  nation,  numbers  its  centuries, 
it  may  be  its  thousands  of  years;  the  continents 
and  islands  which  its  dominion  includes  have  per- 
haps their  date,  as  those  which  preceded  them  have 
had ;  and  the  very  revolutions  of  the  sky,  by  which 
centuries  are  numbered,  will  at  last  languish  and 
stand  still." 

The  final  obliteration  of  the  system,  at  least  so  far 
as  the  comets  are  concerned,  was  a  favorite  theory 
advanced  by  Newton,  and  one  which,  to  a  certain 
degree,  he  seems  to  have  cherished  to  the  latest 
hours  of  his  life.  He  thus  regarded  the  comets  as 
"the  aliment  by  which  suns  are  sustained";  and 
remarked,  that  although  it  was  impossible  for  him 
to  conjecture  when  the  comet  of  1680  —  one  to 
which  he  had  devoted  special  attention  —  will  fall 
to  the  sun,  yet,  whenever  that  time  shall  arrive,  the 
heat  of  the  sun  will  be  raised  by  it  to  such  a  degree, 


STABILITY  OF  THE  SOLAR  SYSTEM.      355 

that  our  earth  will  be  so  intensely  heated,  that  all 
animal  and  vegetable  existence  will  be  destroyed. 
It  has  also  been  urged  that  the  assumption  of  a 
resisting  medium  as  resulting  in  the  final  destruction 
of  our  system,  indicates  a  direct  analogy  between 
the  laws  which  govern  all  parts  of  the  material 
universe  ;  that  perpetual  change  and  perpetual  pro- 
gression, increase  and  diminution,  are  found  thus 
to  prevail  without  exception.  It  must  be  remarked, 
however,  that  this  view  of  the  system  of  the  world, 
although  supported  by  close  analogy,  does  not  on 
that  account  present  unavoidable  claims  to  be  re- 
garded as  strictly  true,  or  even  as  being  anything 
more  than  a  mere  speculation.  It  would  indeed 
seem  strange  and  unaccountable  that  the  Omnipo- 
tent Creator  of  the  universe  should  construct  systems 
of  worlds  revolving  around  a  common  centre,  in 
accordance  with  laws  so  regulated,  if  not  subjected 
to  external  causes,  as  to  perpetuate  each  individual 
and  compound  system.  There  may  be  a  beginning 
without  an  end,  although  the  reverse  is  impossible, 
at  least  so  far  as  human  reason  can  understand; 
and  to  conceive,  therefore,  that  because  the  present 
state  of  things  has  had  a  beginning,  it  must  neces- 
sarily have  an  end,  is  by  no  means  admissible.  Nor 
is  it  enough  to  say  that  the  resisting  medium  does 
not  in  the  least  counteract  what  is  most  important 
in  the  provision  for  the  perpetuity  of  our  system, 


356  TREATISE   ON    COMETS. 

and  that  the  order  of  nature  will  remain  unchanged 
for  a  period,  compared  with  which  the  age  of  the 
earth  in  its  present  state  is  insignificant.  The  ulti- 
mate effect  of  the  resisting  medium  will  be,  as 
already  noticed,  to  dimmish  not  only  the  periods  of 
the  revolutions  of  the  different  planets  and  comets, 
but  also  that  of  the  earth,  and  thus  to  derange  those 
adaptations  which  depend  on  the  length  of  the  year, 
and  fit  it  for  the  abode  of  animal  life  such  as  now 
exists.  It  may,  however,  have  been  preordained 
that  our  own  system,  at  least,  should  exist  for  a 
specific  purpose,  and  that,  having  fulfilled  its  pur- 
pose, it  should,  as  such,  have  its  end  of  duration, 
though  millions  of  millions  of  years  were  designated 
by  the  Creator  as  the  period  in  which  all  its  suc- 
cessive transformations  should  be  effected.  When 
the  present  order  of  things  had  its  beginning  is 
beyond  the  power  of  the  human  reason  to  declare ; 
and  when,  if  ever,  it  shall  end,  is  equally  beyond 
our  comprehension.  In  all  such  inquiries  we  are 
soon  lost  and  bewildered  by  the  uncertainties  which 
surround  us,  and  entering,  as  we  do,  the  confines 
of  the  unknown,  it  becomes  as  impossible  to  proceed 
with  the  investigation  as  to  attempt  to  conceive  of 
the  origin  of  Omnipotence  itself.  The  power  of 
the  human  reason  is  finite,  and,  therefore,  unequal 
to  the  comprehension  of  the  infinite;  and,  in  view 
of  this,  we  can  only  hope  that  the  time  will  come, 


INFLUENCE   OF   THE  COMETS.         357 

and  at  no  very  distant  day,  when  some  of  the 
mysteries  in  which  many  of  the  phenomena  of  the 
material  world  are  involved,  will  be  finally  dissipated, 
and  when  the  field  of  speculation,  which,  to  a 
limited  extent,  must  ever  remain,  will  be  finally 
compressed  within  very  narrow  limits. 

In  regard  to  the  theory,  that  there  exist  evident 
indications  that  all  must  end,  it  might  be  added 
that  the  limit  of  even  millions  of  millions  of  ages 
is  by  no  means  suificient  to  dispel  any  tendency  in 
the  mind  to  revolt  against  any  such  assumption. 
Granting,  therefore,  the  existence  of  a  resisting 
medium  in  space,  and  in  view  of  the  ultimate  effect 
which  would  thus  be  produced,  if  not  counteracted, 
we  are  naturally  inclined  to  look  to  the  operation 
of  some  other  force,  yet  unknown,  to  obviate,  or, 
at  least,  modify  the  nature  and  degree  of  the  effect, 
and  thus  avert  the  dreadful  calamity  which  must 
eventually  be  produced.  The  perturbations  of  the 
comets  on  each  other  are  yet  undetermined,  and 
also  the  ultimate  effect  of  the  attractions  of  these 
bodies  on  the  solar  system.  The  phenomena  of  the 
tails  of  comets  have  indicated,  beyond  all  doubt, 
the  existence  of  a  repulsive  force  in  all  material 
bodies,  while  the  fact  that  the  force  of  gravity,  and 
the  undulations  of  the  ethereal  fluid  in  producing 
the  intensity  of  light,  vary  according  to  the  same 
law,  taken  in  connection  with  the  supposed  —  and, 


358  TREATISE   ON   COMETS. 

we  believe,  clearly  established  —  identity  of  light, 
heat,  and  electricity,  affords  cogent  reasons  for  con- 
jecturing that  there  may  exist  some  unknown  bond 
connecting  all  these  forces  of  nature  in  such  a 
manner  that  their  combined  effect  will  be  productive 
of  the  most  harmonious  results.  In  this  way  it  may 
be  considered  possible  that  future  investigations 
will  establish  such  relations  between  the  operation 
of  the  various  physical  forces,  as  will  exhibit  in  the 
clearest  and  most  instructive  manner,  the  evident 
design  of  perpetuity  which  the  Creator  may  have 
enstamped  on  every  part  of  his  universe  of  creation, 
already  ended. 

There  is  also  another  point  of  view  from  which 
the  conditions  of  the  perpetuity  of  the  systems  of 
the  universe  may  be  considered,  which  must  not  be 
passed  by  unnoticed,  and  which  possibly  may  furnish 
an  explanation,  by  no  means  improbable,  of  the 
object  of  the  comets  in  the  economy  of  nature.  If 
we  suppose,  that,  in  the  course  of  an  indefinite 
number  of  ages,  an  almost  infinite  number  of  these 
bodies  shall  pass  through  our  system,  it  may  be 
concluded  that,  assuming  their  individual  masses  or 
weights  to  be  small,  their  disturbing  influence  in 
the  system  will  be  sufficient  to  counterbalance  any 
minute,  or,  so  to  speak,  residual  inequalities  or 
irregularities  which  may  exist.  We  may  suppose 
from  analogy  —  and  there  can,  in  this  case,  be  no 


GENERAL  KEMARKS.  359 

forcible  objection  to  such  a  course  of  reasoning  — 
that,  in  the  other  systems  of  the  universe,  there 
exist  the  same  conditions  and  arrangements  for 
preserving  their  stability  as  in  the  case  of  our  own 
system.  From  what  we  know  of  the  motions  of 
the  comets,  after  they  cease  to  be  visible  from  the 
earth  and  in  the  remote  regions  of  space,  it  may  be 
inferred  that  they  pass  through,  and  present  in  other 
systems  the  same  phenomena,  only  to  a  greater  or 
less  extent,  which  have  characterized  their  appear- 
ance in  our  own ;  and  that  although  some  of  them 
will  be  drawn  for  a  time,  at  least,  into  orbits  which 
would  retain  them  as  members  of  the  system,  yet 
by  far  the  greater  number  would  move  in  orbits 
which  would  conduct  them  beyond  the  confines  of 
the  various  systems  which  they  may  visit ;  and  so, 
passing  on  in  a  meandering  course,  from  sun  to 
sun,  and  from  system  to  system,  they  may  poise 
even  by  their  infinitesimal  masses,  as  circumstances 
may  require,  every  minute  irregularity  in  the  sys- 
tems which  they  visit,  thus  preserving,  through 
time  and  eternity,  the  stability  and  equilibrium  of 
the  universe. 


Such  is  the  present  state  of  cometary  astronomy, 
divested,  so  far  as  practicable,  of  its  technicalities, 
and  brought  within  the  comprehension  of  every 


360  TREATISE   ON   COMETS. 

general  reader.  The  phenomena  which  have  been 
presented  are  such  as  furnish  the  most  sublime 
objects  for  contemplation;  and  in  what  has  been 
stated,  whether  on  the  solid  basis  of  established 
truth,  or  in  the  form  of  a  speculation,  we  have 
endeavored  to  be  careful  to  avoid  everything  which 
might,  should  the  train  of  thought  be  continued 
further,  lead  to  erroneous  conclusions.  We  have 
also  endeavored  to  give  all  the  most  important  facts 
which  the  researches  of  astronomers  have  developed 
in  connection  with  the  theory  of  these  wonderful 
bodies,  and  have  shown  conclusively,  as  must  be 
admitted,  that  these  objects  have  no  direct  influence 
in  the  local  affairs  of  our  earth.  We  have  not 
alluded  to  any  supposed  possibility  of  the  comets 
being  regarded  as  the  abode  of  animal  or  vegetable 
life,  except  in  the  statement  that  Winston  supposed 
these  bodies  to  be  the  residence  of  the  damned, 
who  are  punished  by  being  alternately  wheeled  into 
regions  of  intolerable  heat  and  of  the  most  intense 
cold.  It  should  not,  however,  be  supposed  that, 
among  the  various  speculations  respecting  the  cha- 
racter of  these  bodies,  they  have  never  been  re- 
garded as  the  abode  of  rational  and  intelligent 
beings.  Lambert,  in  contemplating  the  excursions 
which  these  bodies  make  in  the  immensity  of  space, 
concludes  with  the  following  reflections,  which  must 


GENERAL  REMARKS.  361 

necessarily  be  regarded  as  simply  a  fanciful  con- 
ception of  a  brilliant  imagination : 

"I  love  to  figure  to  myself  those  travelling  globes, 
peopled  with  astronomers,  who  are  stationed  there 
for  the  express  purpose  of  contemplating  nature  on 
a  large,  as  we  contemplate  it  on  a  small,  scale. 
Their  movable  observatory  cruising  from  sun  to 
sun,  carries  them  in  succession  through  every  dif- 
ferent point  of  view,  places  them  in  a  situation  to 
survey  all,  to  determine  the  position  and  motion  of 
each  star,  to  measure  the  orbits  of  the  planets  and 
comets  which  revolve  around  them,  to  observe  how 
particular  are  resolved  into  general  laws,  in  one 
word,  to  get  acquainted  with  the  whole  as  well  as 
the  detail.  We  may  suppose  that  their  year  is 
measured  by  the  length  of  their  route  from  one  sun 
to  another.  Winter  falls  in  the  middle  of  their 
journey;  each  passage  of  a  perihelion  is  the  return 
of  summer;  each  introduction  to  a  new  world  is 
the  revival  of  spring ;  and  the  period  of  quitting  it 
is  the  beginning  of  their  autumn.  The  place  of 
their  abode  is  accommodated  to  all  their  distances 
from  the  fixed  stars,  and  the  different  degrees  of 
their  heat  make  the  fruits  and  vegetables  designed 
for  their  use  blossom  and  ripen.  Happy  intelli- 
gences, how  excellent  must  be  the  frame  of  your 
nature !  Myriads  of  ages  pass  away  with  you  like 
so  many  days  with  the  inhabitants  of  the  earth. 
31 


362  TREATISE   ON    COMETS. 

Our  largest  measurements  are  your  infinitely  small 
quantities;  our  millions  the  elements  of  your  arith- 
metic ;  we  breathe  but  a  moment ;  our  lot  is  error 
and  death,  yours  science  and  immortality.  All  this 
is  agreeable  to  the  analogy  of  the  works  of  creation. 
The  frame  of  the  universe  furnishes  matter  of  con- 
templation as  a  whole  as  well  as  in  each  of  its  parts. 
There  is  not  a  point  that  does  not  merit  our  obser- 
vation ;  this  magnificent  fabric  is  portioned  out  in 
detached  parts  to  created  beings ;  but  it  is  in  the 
unity  of  the  whole  that  sovereign  perfection  shines ; 
and  can  we  suppose  that  this  whole  has  no  ob- 
servers ?  The  imagination,  indeed,  after  so  sublime 
a  flight,  may  be  astonished  at  its  own  temerity; 
but,  in  short,  here  the  cause  is  proportioned  to  the 
effect,  and  there  is  nothing  great  or  small  in  im- 
mensity and  eternity." 

Whether  the  comets  are  in  a  state  capable  of 
being  the  abode  of  rational  intelligences,  or  whether, 
as  is  more  probable,  they  are  only  exhibitions  of 
matter  in  a  state  of  transformation,  are  questions 
which  we  shall  not  consider.  They  are  by  no 
means  necessary  to  a  complete  understanding  of 
our  subject,  and  for  this  reason  have  been  omitted. 
We  may,  therefore,  remark  in  conclusion,  that 
enough  has  been  presented  to  almost  overwhelm 
the  mind  in  the  contemplation  of  the  material  uni- 
verse. 


GENERAL  REMARKS.  363 

Well  hath  the  great  Creator  of  the  world, 
Fram'd  it  in  that  exact  and  perfect  form, 
That  by  itself  immovable  might  stand, 
Supported  only  by  his  providence. 
Well  hath  his  powerful  wisdom  ordered 
The,  in  nature,  disagreeing  elements, 
That  all  affecting  their  peculiar  place, 
Maintain  the  conservation  of  the  whole. 
Well  hath  he  taught  the  swelling  ocean 
To  know  its  bounds,  lest  in  luxurious  pride 
He  should  insult  upon  the  conquer' d  land: 
Well  hath  he  plac'd  those  torches  in  the  heavens, 
To  give  light  to  our  else  darkened  eyes: 
The  crystal  windows  through  which  our  soul, 
Looking  upon  the  world's  most  beauteous  face, 
Is  blest  with  sight  and  knowledge  of  his  works." 


THE    END. 


14  DAY  USE 


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